NGC 2362 is a small compact young open cluster in Canis Major that surrounds bright star Tau Canis Majoris (τ CMa - mag. +4.37). This attractive grouping of 60 stars is packed into an area spanning just 6 arc minutes of apparent sky. The apparent magnitude of the cluster is given as +4.1, however the value is misleading as its skewed significantly because of the brilliance of τ CMa. The remaining members of NGC 2362 are much fainter, the brightest being of 7th magnitude.
Finding NGC 2362 is not difficult. It's located 2.75 degrees northeast of Wezen (δ CMa - mag. +1.83) the third brightest star in Canis Major. Sirius (α CMa) the brightest star in the night sky (mag. -1.46) is positioned 11 degrees to the northwest. Tau CMa a spectroscopic multiple system that shines with a combined light of tens of thousands times that of the Sun is the stand out cluster member and bright enough to be seen with the naked eye. For comparison, the Sun at the same distance would shine at a feeble magnitude +15.
NGC 2362 was discovered by Giovanni Batista Hodierna sometime before 1654 and then re-discovered by William Herschel on March 4, 1783. It's best seen from southern latitudes during the months of December, January and February.
NGC 457 is the brightest open cluster in Cassiopeia and one of the finest objects of its type in the northern sky. At magnitude +6.4, it's just beyond naked-eye visibility but easily seen with binoculars and a beautiful sight through telescopes. The brightest cluster stars are arranged in prominent lines and curves lines appearing to resemble an Owl shape, hence the popular name "the Owl Cluster". It's located 7,900 light-years distant.
NGC 457 was discovered by William Herschel in 1787. Finding the Owl Cluster is easy; it's positioned two degrees south-southeast of eclipsing binary star system Ruchbah (δ Cas - mag. +2.7). This star is one component of the characteristic "W" asterism of Cassiopeia. The brightest star inside NGC 457 is Phi Cas (φ Cas - mag. +5.0). Despite not being a member of the cluster, this foreground star is visible to the naked eye. Together with another non-cluster star - seventh magnitude HD 7902 (HIP 6229) - they form the bright eyes of the Owl greatly adding to the splendour of the view through backyard scopes.
NGC 457 is best seen from Northern Hemisphere latitudes during August, September and October. It appears high in the sky and even overhead from many locations. From latitudes greater than 32N, the Owl is circumpolar and never sets.
NGC 752 is a large spawling open cluster in the constellation of Andromeda. With an apparent magnitude of +5.7, it's visible to the naked eye from a dark site appearing as a large unresolved fuzzy patch of light. The cluster is one of the finest large open clusters in the sky and contains over 70 stars spread across a huge 1.25 degrees of apparent sky. Due to its size, NGC 752 is best observed with binoculars or through wide field telescopes at low powers.
NGC 752 is located 5 degrees south and slightly west of outstanding double star Almach (γ And - mag. +2.1) the third brightest star in the constellation. It was discovered by Caroline Herschel on September 29, 1783 although it was probably observed sometime before 1654 by Italian astronomer Giovanni Battista Hodierna. Caroline's brother William Herschel subsequently added it to his catalogue a couple of years later. It's best seen from northern latitudes during the months of October, November and December. The cluster is number 28 in the Caldwell catalogue.
NGC 2775 is a magnitude +10.5 spiral galaxy located in the constellation of Cancer, close to its border with Hydra. The galaxy is unusual in that it contains a very smooth nucleus with multiple spiral arms extending outwards from the central region. What makes the spiral arms interesting is their incredible complex detail, tightly wound structures and active star formation. Amateur astronomers should also keep their eye on this galaxy; it's been host to 5 supernovae explosions in the past 30 years and you never know when the next one will go off!
To find NGC 2775 look for the head of Hydra "the Sea Serpent". The asterism of stars that forms the head are ω Hyd (mag. +5.0), ζ Hyd (mag. +3.1), ρ Hyd (mag. +4.4), ε Hyd (mag. +3.4), δ Hyd (mag. +4.1), σ Hyd (mag. +4.5) and η Hyd (mag. +4.3). None of the stars are particularly bright but all can be seen with the naked eye. The galaxy is positioned a few degrees east and slightly north of this grouping.
NGC 2775 was discovered by William Herschel in 1783 and is best seen during the months of February, March and April. The galaxy is located 55.5 million light-years from Earth and has an actual diameter of 70,000 light-years. It's estimated to contain 100 billion stars. NGC 2775 is number 48 in the Caldwell catalogue.
NGC 2477 is a stunning open cluster located in the Milky Way rich constellation of Puppis. It's arguably the constellations finest cluster which also contains other superb examples such as M46, M47 and M93. At magnitude +5.8, NGC 2477 is faintly visible to the naked eye but easily seen with binoculars and a fantastic telescope object, especially in medium to large scopes. It's listed as number 71 in the Caldwell catalogue.
The cluster was discovered by French astronomer Nicolas Louis de Lacaille during his tour of South Africa in 1751-52. In total it contains about 300 stars packed into an area 27 arc minutes in diameter with the brightest member star shining at magnitude +9.8. The four-magnitude difference between the combined cluster magnitude and the brightest component is an indication of how rich the cluster is.
NGC 2477 is too far south to have been included in Charles Messier's catalogue, but if he had observed from a more southerly latitude than Paris he almost certainly would have noticed this striking object. Twentieth century America astronomer Robert Burnham described NGC 2477 as "probably the finest of the galactic clusters in Puppis".
The cluster is easily found 2 degrees northwest of zeta Pup (ζ Pup - mag. +2.2) and just northwest of magnitude +4.5 star, b Pup. Located 1.5 degrees west of NGC 2477 and in the same binocular field of view lies large loose open cluster NGC 2451. Another binocular open cluster, NGC 2546, is positioned 4 degrees east of NGC 2477. They are best seen from southern latitudes during the months of December, January and February.
NGC 40 is a planetary nebula located in the northern constellation of Cepheus. It was discovered by William Herschel on November 25, 1788 who described it as "a 9th magnitude star, surrounded with milky nebulosity". Herschel used his 475mm (18.7) inch telescope to make the discovery but for today's amateur astronomers such a large scope isn't required, it can be glimpsed with just a 100mm (4-inch) instrument. NGC 40 is also known as the Bow Tie nebula, a nickname it shares with another planetary nebula, NGC 2440 in Puppis. It's listed as number 2 in the Caldwell catalogue.
NGC 40 is located just over 17 degrees from the North Celestial Pole and therefore circumpolar from most northern latitudes. It's one of the finest examples of its type in the far northern part of the sky. The best time to look for the nebula is during October, November and December when it appears highest in the sky during early evening. The Bow Tie nebula is also visible from most tropical locations although lower down. However, from southern temperate latitudes it's not visible at all.
Locating NGC 40 can be precarious as it's positioned in a star poor region of eastern Cepheus. One method to find it is by imagining a line connecting Errai (γ Cep - mag. +3.21) and γ Cas (mag. +2.15). The planetary lies approximately one-third of the way along this line.
NGC 188 is an open cluster located in the far northern constellation of Cepheus. It was discovered by John Herschel - the son of William Hershel - on November 3, 1831. He originally recorded it as h 34 in his 1833 catalogue and then included it as GC 92 in his subsequent General Catalogue of 1864. The cluster finally became NGC 188 in John L.E. Dreyer's New General Catalogue of 1888.
NGC 188 is the northernmost open cluster in the sky, it's positioned only 4.75 degrees from the North Celestial Pole. Located at such a northerly declination means the cluster is circumpolar from almost the entire northern hemisphere. It can be seen from the Southern Hemisphere but only from latitudes north of 5 degrees south. Even then it never climbs more than a few degrees above the northern horizon.
Although comet Lovejoy has faded significantly since it reached naked eye brightness a few months ago it should still remain an easy telescope target during May. During the month the comet continues on its northerly path heading towards a close pass of Polaris and the North Celestial Pole.
Location and star chart
Lovejoy spends the first couple of days of May in northern Cassiopeia before moving into Cepheus where it remains until the 28th. The surrounding star field is faint and there's not much in the way of deep sky objects but on the 20th Lovejoy passes 0.5 degrees east of open cluster NGC 188. With an apparent magnitude of +8.1, NGC 188 or Caldwell 1 is not one of the sky's brightest clusters but can be seen with binoculars, although much easier with telescopes. Positioned less than 5 degrees from the North Celestial Pole it holds the distinction of being the northernmost open cluster in the sky. When passing the cluster current predictions put Lovejoy at magnitude +8.5 and therefore similar in brightness.
The next major pass for Lovejoy occurs on May 29th when it passes just 1 degree west of Polaris (α UMi - mag. +2.0). It's likely to have faded to 9th magnitude by this time. As a consequence of it's high northerly declination the comet is circumpolar from virtually all the Northern Hemisphere but can't be seen from Southern latitudes.
The finder charts below show the positions of Lovejoy from March 12th to June 12th, 2015.
Comet MASTER remains well placed from southern latitudes as it heads towards closest approach to Earth and perihelion in May. During the first half of the month it's visible towards the east just before sunrise, before switching to the evening sky for the last two weeks. It's then appears fairly high in the western sky after sunset. At its brightest MASTER should be visible to the naked eye and easily seen with binoculars.
Unfortunately, from northern temperate locations the comet is not visible until September when it will be extremely faint.
Location and star chart
The comet picks up pace during May as it moves through a handful of southerly constellations. It starts the month in Sculptor on an east-southeast trajectory before crossing into Fornax on May 8th. The next stop for MASTER is Eridanus where it arrives on May 14th. Three days later it briefly passes through Caelum before moving into Lepus on May 18th. Then comes Canis Major on May 22nd and finally the comet crosses into Monoceros on May 26th where it remains for the rest of the month.
It's predicted that MASTER will brighten from magnitude +7.0 at the beginning of the month to a peak of magnitude +5.1 on May 15th. A few days either side of this it should be visible to the naked eye and an easy binocular and small scope target. On May 18th, the comet passes less than a degree north of mag. +8.1 globular cluster M79 in Lepus. Although fading to magnitude +6.3 by months end it remains bright and easily within binocular and small scope range. Of course, comets are unpredictable and anything could happen, so keep watching!
MASTER passes closest to Earth on May 13th at 70 million kilometres or 44 million miles distant before reaching perihelion on May 23rd when it's 115 million kilometres (72 million miles) from the Sun.
The finder charts below show the positions of comet MASTER from April 17 to May 26, 2015.
Of all the planets in the Solar System the most beautiful of all is Saturn. On May 23rd the favourite planet reaches opposition and is therefore visible all night. Saturn starts the month continuing its retrograde motion in Scorpius before slowly moving into faint Libra on May 12th where it remains for the rest of the month. With a declination of –18 degrees it's better placed for Southern Hemisphere or tropical based observers where it appears higher in the sky and visible for a longer period of time.
At opposition, Saturn shines at magnitude +0.1 and is located 8.967 AU or approximately 1341 million kilometres (833.5 million miles) from Earth. Of course the spectacular rings are its most famous feature and even a small telescope will show them. They are currently wide open at a 24.4 degree tilt from our perspective. With an apparent diameter of 18.5 arc seconds and taking into account the rings, Saturn spans some 42 arc seconds across in total.
Through medium and large aperture scopes the rings are a fantastic breathtaking sight. In addition, a handful of Saturn's moons are also visible. The largest and brightest Titan shines at eight magnitude and can be seen with binoculars. In addition, small size scopes will also show other moons including Rhea, Tethys and Dione. Around opposition is a good time to look for moons of Saturn as they are also at their brightest for the year.
A good opportunity exists to spot bizarre moon Iapetus just before opposition. This world is famous for its "two-tone" colouration with one side being much darker in colour than the other. As a result, Iapetus when positioned on the western side of Saturn (when viewed from Earth) appears brighter than from the opposite side. This occurs on May 20 when Iapetus is at greatest western elongation and shines at magnitude +10.1, easily within the range of small scopes.
The chart below shows the position of Saturn in Libra at opposition. It's co-ordinates are R.A. = 15hr 59m 05s, Dec. = –18d 18h 12s.
The first three weeks of May offers an excellent opportunity for observes at Northern Hemisphere and tropical latitudes to spot elusive Mercury. During this apparition the planet is visible just after sunset low down above the west-north-western horizon. Peak altitude occurs on May 7th when greatest eastern elongation is reached. On this day, Mercury will be 21 degrees from our star and visible as a magnitude +0.2 point of light 10 or so degrees above the horizon, 45 minutes after sunset.
Due to the angle of the ecliptic, an apparition of Mercury is often better seen from one particular hemisphere. For this apparition, the Northern Hemisphere wins and this also happens to be the their most favourable evening apparition of the year.
Mercury remains an early evening object for observers located at tropical and Northern Hemisphere latitudes during the first three weeks of May. The planet is located in Taurus and is visible after sunset towards the west-northwest as soon as it's dark enough.
Mercury climbs higher in the sky each evening until it peaks on May 7th, the date of greatest eastern elongation. For example, from 52N (e.g. London, England) the fast moving planet (mag. +0.2) is positioned 11 degrees above the horizon, 45 minutes after sunset. Afterwards its altitude decreases until it's finally lost to the bright twilight sometime during the third week of the month. It should be noted that Mercury is at its brightest before elongation - it fades from magnitude -0.4 to +1.5 during the first half of May. Located a few degrees northeast of Mercury is much more brilliant Venus (mag. -4.3).
The chart below shows positions of Mercury and Venus from latitude 52N (e.g. London, England). The view will be similar from other northern temperate locations.
On May 12th, Mercury (mag. +1.0) passes 8 degrees north of orange giant star Aldebaran (α Tau - mag. +0.9) the brightest star in Taurus. The fast moving planet then reaches a stationary point on May 19th. Retrograde motion follows pulling Mercury back towards the Sun with it arriving at inferior conjunction on May 30th.
From the Southern Hemisphere the planet is not well placed for observation but may be glimpsed extremely low down above the west-northwestern horizon just after sunset at the start of the month.
Venus continues as a blazing evening object as it heads towards greatest eastern elongation of 45 degrees on June 6th. The planet is so bright that it can't be mistaken and can be spotted even before the Sun dips below the horizon. During May it brightens from magnitude -4.2 to -4.4 with its apparent size increasing from 17 to 22 arc seconds. The phase of the planet decreases from 67 to 54 degrees during the same period.
The period of visibility of Venus varies considerably depending on location. From northern temperate latitudes the planet sets almost 3.5 hours after the Sun at the start of the month, increasing to 4 hours by months end. However, much further south the planet can be seen for as little as 2 hours. On May 21st the waxing crescent Moon passes 8 degrees south of Venus and at the end of the month, Venus passes 4 degrees south of the Gemini twins, Castor (α Gem - mag. +1.6) and Pollux (β Gem - mag. +1.1).
Mars has now just a few weeks to go before reaching solar conjunction on June 14th. The "Red planet" spends the first few days of May in Aries before moving into Taurus where it remains for the rest of the month. With an apparent magnitude of only +1.5 it's not bright easily lost to the evening twilight and unlikely to be seen this month.
Jupiter is now 3 months past opposition but despite fading in brightness the planet remains a brilliant object in Cancer. The gas giant is moving direct and positioned 7 degrees southeast of beautiful binocular and small telescope open cluster M44 "The Praesepe".
Jupiter is visible as soon as it's dark enough. At the end of the month, it sets just after midnight from northern temperate latitudes but visibility is somewhat shorter for those living in the tropics and further south. The planets magnitude decreases from -2.1 to -1.9 during May with the apparent size diminishing from 38 to 35 arc seconds over the same time period.
On May 24th, the waxing crescent Moon passes 5 degrees south of Jupiter.
Saturn reaches opposition on May 23rd and is therefore visible all night as it continues its slow retrograde motion, starting the month in Scorpius before moving into faint Libra on May 12th. The favourite planet of many astronomers is now at it's stunning best with the rings wide open at a tilt of 24.4 degrees and an apparent diameter of 18.5 arc seconds. Including the rings it spans some 42 arc seconds across. The current southerly declination of Saturn means it's much better placed for Southern Hemisphere or tropical based observers.
Through telescopes, Saturn's rings are a wonderful sight and visible even in the smallest of instruments along with Titan, the largest and brightest moon of Saturn. Larger telescopes reveal subtle planet details and many of the planets other moons.
At opposition, Saturn shines at magnitude +0.1 and is positioned 8.967 AU or approximately 1341 million kilometres (833.5 million miles) from Earth. The full Moon passes 2 degrees north of Saturn on May 5th.
Uranus at magnitude +5.9 is currently located in Pisces. For the first half of May from northern temperate latitudes the planet is too down low to be seen against the bright morning twilight. However, the situation quickly improves and by months end Uranus rises over 2 hours before the Sun, therefore visible through binoculars and telescopes against the darker backdrop. Observers located further south have it even better with Uranus well placed in the early morning sky throughout the month. At the start of the May, the planet rises 2 hours before the Sun with the visibility period more than doubling by May 31st.
The waning crescent Moon passes 0.2 degrees north of Uranus on May 15th and an occultation is visible from central South America, west and central Africa (11:55 UT).
Neptune is a morning object amongst the faint stars of Aquarius, the Water Bearer or Water Carrier. The planet is better placed from tropical and southern latitudes, rising in the east some four hours before the Sun at the beginning of the month. By months end it can be seen around midnight. From Northern temperate latitudes Neptune is better seen towards the end of the month when it rises 3 hours or so before the Sun.
At magnitude +7.9, the Solar System's most distant planet is never bright enough to be visible to the naked eye but can be spotted with binoculars and small telescopes. Telescopes at medium to high magnifications reveal a featureless small blue disk that spans just 2.3 arc seconds across.
The last quarter Moon passes 4 degrees north of Neptune on May 12th.
The annual Eta Aquariids (or Eta Aquarids) meteor shower peaks on May 6, 2015. Sadly this year's event will be adversely impacted due to the presence of the nearby almost full Moon.
The parent body for the Eta Aquariids shower is Halley's comet (1P/Halley). This isn't the only annual meteor shower associated with the famous comet; the stronger October Orionids also originate from Halley. Although Halley has now left the inner Solar System and won't return until 2061, it's worth remembering that all Eta Aquariids meteors are actually a small part of the famous comet burning up in the Earth's atmosphere!
The radiant of the Eta Aquariids is located in northern part of the zodiac constellation of Aquarius, close to its border with Pegasus and Pisces. The radiant is positioned only 1 degree south of the Celestial Equator and therefore visible from practically all of Earth's surface, exception being the North Pole.
Despite it's global presence the Eta Aquariids meteor shower favours certain locations. The radiant is very well placed from equatorial latitudes, rising in the east after midnight and high in the sky before sunrise. From Southern Hemisphere latitudes it's also well seen but not so good from northern temperate locations. For example, from London, England (51.5N) the radiant rises just before dawn and therefore all but the brightest shooting stars are lost to the morning twilight.
There are at least 6,000 stars in the sky that are bright enough to be visible with the naked eye. Of these we could expect 3,000 or so to be seen at any one time under dark skies (since one half of the Earth is in daylight). However, none can rival the glory of the brightest night-time star of all, Sirius.
Located in the constellation of Canis Major "the Great Dog", Sirius shines with an apparent magnitude of -1.46. It's easily brighter than its nearest rival Canopus (α Car mag. -0.72) and four times more brilliant than Arcturus, the brightest star in the northern section of sky. Sirius is also known as the "Dog Star" and what's immediately noticeable is it twinkles! The star flashes through many colours of the rainbow - especially when close to the horizon - but of course this has nothing to do with Sirius itself; it's a pure white star and the colour variations are solely caused by the Earth's unsteady atmosphere. In practice all stars do twinkle, also to a lesser extent the planets, but the effect is most obvious with Sirius.
To the naked eye Sirius appears as a single star but it's actually a binary system. The primary component is a white main-sequence star of spectral type A1V termed Sirius A. This star - the one we see with the naked eye - has a radius of 1.7 times that of the Sun and is 25 times more luminous than our star. However, the secondary component is much fainter.
In 1844, German astronomer Friedrich Bessel noticed that Sirius "weaved" its way through the sky instead of moving in a straight line. He deduced that there must be an unseen companion at least as massive as the Sun and with an orbital period of 50 years. Bessel worked out where the companion should be but despite extensive observations he couldn't find it.
It was on January 31, 1862 when American telescope-maker and astronomer Alvan Graham Clark finally discovered the companion, now named Sirius B or the "Pup". The reason why it took so long was because it's a white dwarf star, 10,000 dimmer than its neighbour. Clark discovered Sirius B with an 18.5-inch (470 mm) aperture refractor telescope, which happened to be the largest refracting telescope in the World at that time. At magnitude +8.6, Sirius B isn't particularly faint and if located on it's own, it would even be visible with binoculars.
Today, Sirius B can be spotted with a much small sized scope than Clark's. The separation between the two stars varies between 3 and 11.5 arc seconds. It's even possible to glimpse the Pup with a good quality 100mm (4-inch) refractor on nights of excellent seeing when close to maximum separation. When at minimum separation the B star is incredibly difficult to observe.
In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has a diameter of 12,000 kilometres (7,500 miles) almost equal to that of the Earth and a mass 98% that of the Sun. The star system is close to Earth at 8.6 light-years distant. Perhaps surprising, Sirius is one of the least powerful first magnitude stars, its brilliance is simply due to its proximity.
Naturally Sirius has its place in ancient history. It's name has Greek origins which is unusual as most other named stars are Arabic based. The Greek translation is "sparkling" or "scorching" and along with the Romans they regarded it as an unlucky omen. The Egyptians placed much significance on Sirius. It was worshipped as Sothis or the "Nile Star" and it's first appearance in the dawn sky marked the important annual flooding of the Nile River.
With a declination of 16S, Sirius is visible from almost the entire World. The exceptions are latitudes 74N or greater where the star never rises above the southern horizon. However, from cities just inside the Artic Circle such as Murmansk in Russia, Tromsø in Norway and Barrow in Alaska it can be seen, though admittedly not for very long.
Finding Sirius is easy but in case of uncertainty it's located along an imaginary line extending from the three stars of Orion's belt southwards.
There is an old mystery surrounding the colour of Sirius. It was often described in ancient times as red or orange although today it's clearly pure white. Around AD 150, the great astronomer of his time Ptolemy described Sirius as reddish along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux. Some astronomers agreed with Ptolemy's observations but others noticed only a white star. However, it all seems very strange.
Changes to Sirius A or B over a period of 2,000 years can be rejected as the timescale is too short and there is no sign of nebulosity in the system that would be expected had such a change taken place. Sir John Herschel suggest that a space cloud may have passed between us and Sirius therefore reddening the light as seen from our perspective but this seems unlikely as we would almost certainly be able to still detect such a cloud today. It also seems unlikely that the twinkling of Sirius would have fooled Ptolemy as any red colour is never permanent and the star was relatively high in the sky from his location and above the thicker layers of the atmosphere.
Most likely there was no change in the colour of the star but this remains a curious mystery even today!
Newly found comet MASTER (C/2015 G2) is currently brightening sufficiently that it should reach naked eye brightness from southern and equatorial latitudes soon. When discovered on April 7th by astronomers using the MASTER-SAAO telescope at the South African Astronomical Observatory near Sutherland it shone at magnitude +10.7. The following week it had already brightened to 9th magnitude and is now on course to reach magnitude +5 during the middle of May.
Location and star chart
Comet MASTER spends most of April moving through Aquarius on a south-easterly trajectory. On April 20th it passes 0.5 degrees north of planetary nebula NGC 7293, the Helix Nebula. At magnitude +7.3, the Helix isn't visible to the naked eye but is still the sky's brightest planetary nebula. However, it's a large object with a low surface brightness and therefore not as easy to spot as it's magnitude suggests. For example, the Dumbbell Nebula (M27), a slightly fainter but more compact planetary is much easier to see.
The comet then cuts through a corner of Piscis Austrinus on April 27th when it's located 5 degrees north and slightly east of the constellations stand out star, Fomalhaut (α PsA - mag. +1.2). After spending only a few hours in Piscis Austrinus it then moves into faint Sculptor where it remains for the rest of the month.
At the end of April, MASTER is predicted to have brightened to magnitude +6.8. On May 13th it reaches closest point to Earth at 70 million kilometres or 44 million miles distant. The comet should continue to brighten before peaking at magnitude +4.9 on May 15th. It then reaches perihelion on May 23rd when it's located 115 million kilometres (72 million miles) from the Sun.
From southern and equatorial latitudes, MASTER is currently well placed for observation and is visible towards the east before sunrise during April. From northern temperate locations the comet is not observable until September when it will be extremely faint. Of course, comets are unpredictable and anything could happen, so keep watching!
The finder chart below shows the positions of comet MASTER from April 17 to May 5, 2015.
Arcturus, mag. -0.04, is an orange giant that's usually regarded as the fourth brightest star in the night sky. However, it does have justifiable claims for third position since it's marginally brighter than both main components of the Alpha Centauri system. What's clear is that Arcturus is the brightest star in the northern section of the celestial hemisphere.
Arcturus is the stand out star in the large constellation of Boötes, the Herdsman or Plowman. A vague legend has it that the herdsman was placed in the heavens for successfully inventing the plough. The constellations next brightest star Izar (ε Boo) shines much fainter than Arcturus at magnitude +2.35. As one of the nighttime's brightest stars Arcturus has been significant to observers since antiquity. The star is mentioned in the Bible and was featured on old Chinese star maps, named Dajido. In India it was sometimes referred to as Nishtya or the Outcast, presumably because of its position in the sky far away from the zodiac and Milky Way band.
The name Arcturus derives from Arktouros, which means in ancient Greek the "Bear's Tail" or equally could refer to "the Keeper of the Bear". The name in Greek literature goes back to at least the time of Hesiod, who wrote about the star in his book "Works and Days." Despite Arcturus being a beautiful star it has not always been held in high regard. For example, seamen of ancient times regarded it as an unlucky omen.
More recently, Ptolemy called it "golden red" and curiously in 1852 some well-respected astronomers observed a change in the star's colour before it reverted back to normal a few years later. It's difficult to believe any intrinsic changes to Arcturus had occurred - it's not that type of star - and it's likely that the colour changes were due to atmospheric effects.
Finding the star is easy, just follow the tail of the Plough or Big Dipper asterism of Ursa Major southwards to arrive at the distinct orange hue of Arcturus. Located at a declination of 19N it's visible from every inhabited location on Earth except from Antarctica science stations. It appears highest in the sky from tropical and Northern Hemisphere latitudes during the months of April, May and June.
Arcturus is a giant type K0 III star with a diameter 25 times larger than the Sun. If placed at the centre of the Solar System it would extend more than half the way to the orbit of Mercury. Although large, it's nowhere near as vast as supergiant stars such as Betelgeuse (α Ori) and Antares (α Sco).
Arcturus is relatively close at 37 light-years distant and 170 times more luminous than the Sun. It's 7.1 billion years old and therefore 2.5 billion years older than our star. The star is single and noted for its high proper motion, two arc seconds a year, greater than any first magnitude star other than alpha (α) Centauri. It's currently at about its closest point to the Sun and to date no planets have been identified surrounding it.
In 1933, the light from Arcturus was used to open the "Century of Progress" exposition in Chicago. The star was selected at that time as it was though to be 40 light-years distant and therefore the light arriving had left the star at the time of the previous exposition.
47 Tucanae or 47 Tuc is a spectacular globular cluster located in the southern constellation of Tucana. At magnitude +4.5, it appears to the naked eye as a slightly fuzzy star similar to the head of a tail-less comet. Always hidden from view for European and North American observers, 47 Tuc was discovered by French astronomer Nicolas Louis de Lacaille on September 14, 1751. Initially Lacaille though he had found a comet until further inspection revealed its true nature.
47 Tuc is the second brightest globular in the sky, only Omega Centauri is more brilliant. It has an extremely dense core and is one of the most massive globular clusters surrounding the Milky Way. The cluster is located 2.5 degrees west of the Small Magellanic Cloud (SMC) and from most of the Southern Hemisphere it's circumpolar and never sets. In contrast from latitudes of 18N or greater, the globular can never be seen as it fails to rise above the horizon.
Through 7x50 or 10x50 binoculars, 47 Tuc appears as a bright starlight nucleus surrounding by a halo of soft pearly light. It's clearly non-stellar in nature. Telescopically the cluster is stunning and a showpiece object of the night sky. It total it spans 31 arc minutes of apparent sky, almost exactly the same diameter as the full Moon. For comparison, 47 Tuc is 50% larger and 3x brighter than M13 "the Great Hercules Globular Cluster" widely regarded as the finest globular in the northern section of the sky.
A small 100mm (4-inch) scope reveals a bright compact core surrounded by a large 15 arc minute sphere with the brightest members resolvable. Even through small telescopes it's a superb sight. A 200mm (8-inch) instrument shows a swarm of stars in a glittering 3D view. The dense centre remains unresolvable in stark contrast to the less dense outer regions. Overall it's a breathtaking object for all sizes and types of telescopes.
47 Tuc is located 16,700 light-years from Earth and contains at least 500,000 stars. These include exotic stars with at least 23 blue stragglers and 23 millisecond pulsars known. The globular is estimated to be 13.1 billion years old.
The 2015 annual Lyrids meteor shower peaks on April 22nd and this year's event promises to be a good one as the four-day-old waxing crescent Moon (21% illuminated) will not interfere. The Lyrids are not one of the strongest annual displays and the peak period is short but up to 20 meteors per hours can be seen. This include occasional fireballs that streak through the sky, casting shadows for a short time and leaving a trail of dust and debris as they disintegrate in the Earth's atmosphere.
The Lyrids radiant is located inside Hercules very near to the border with Lyra and only 6 degrees from the fifth brightest star in the sky, Vega (magnitude 0.0). Unlike sporadic meteors that originate from anywhere in the sky, periodic shower meteors can always be traced back to the same region the radiant point of the meteor shower. Therefore, spotting these shooting stars could not be easier…..just focus on the radiant point? The answer is not so simple. The problem is that although the meteors do originate from the radiant point they can streak across almost any part of the sky! Therefore its best to scan a large area surrounding the radiant without directly looking at it.
The shower activity lasts from April 16th to April 26th with the best time to observe around midnight on the evening of April 22nd / 23rd.
Comet Lovejoy continues on its northern path during April as heads towards Polaris and the North Celestial Pole. This superb comet has put on an excellent performance so far. At peak it was far brighter that originally expected and bright enough to be seen with the naked eye. Through binoculars and telescopes it was a stunning sight. The good news is that although Lovejoy has now faded and not as impressive as previous, it remains within the range of binoculars and small scopes.
Location and star chart
Lovejoy spends all of April in Cassiopeia. At the start of the month it's positioned just north of the famous "W" asterism of the constellations brightest stars. It then continues to move northwards through the barren patch of sky towards Cepheus. During this time, there are many nearby fainter stars that can be easily seen in binoculars for star hopping purposes.
Lovejoy is expected to fade from +6.5 to +8.2 during April. It's circumpolar from almost all the Northern Hemisphere but can't be seen from southern temperate latitudes. Comets are unpredictable so keep watching as you never know what might happen!
The finder charts below show the positions of the comet from February 12th to May 3rd, 2015.
Mercury reaches superior conjunction on April 10th and is therefore too close to the Sun to be seen at this time. However it doesn't take long before the fast moving planet reappears in the early evening sky. From northern and equatorial latitudes, it's visible from about April 21st or so, low down above the west-northwestern horizon just after sunset. For northern observers this also happens to be the most favourable evening apparition of the year.
The following day Mercury (mag. -1.4) passes just over a degree north of Mars (mag. +1.4). Binoculars will show both objects in the same field of view with Mercury 15 times the brighter. A 200mm (8-inch) telescope at medium to high magnification should reveal the disks of both planets although they are both small (Mercury 5.7 arc seconds, Mars 3.8 arc seconds). In the same region of sky and positioned a few degrees to the northeast of the pair is much more brilliant Venus (mag. -4.1).
Mercury then continues to climb higher each subsequent evening, although at the same time fading in brightness, until it reaches greatest eastern elongation on May 7th. From the Southern Hemisphere the planet is nowhere near as well placed for observation but may still be glimpsed extremely low down above the west-northwestern horizon just after sunset towards the end of the month.
The chart below shows positions of Mercury and Venus from latitude 52N (e.g. London, England). The view will be similar from other northern temperate locations.
Brilliant Venus, mag. -4.1, continues to dominate the western sky after sunset. The unmistakable planet is visible as soon as dark enough although the period of visibility varies considerably depending on location. From northern temperate latitudes Venus sets almost 3.5 hours after the Sun at the start of the month, increasing to 4 hours by months end. However, much further south the planet can be seen for as little as 2 hours.
On April 11th, Venus passes 2.5 degrees south of beautiful open cluster the Pleiades (M45). This grouping makes a wonderful pairing for binoculars or wide-field telescopes. Later in the month (April 21st) it's located 7.5 degrees north of Aldebaran and another open cluster, the large sprawling Hyades. On the same evening the waxing crescent Moon passes 7 degrees south of Venus.
The long Mars period of visibility finally comes to an end in April. For sometime now the famous Red planet has kept ahead of the Sun in the early evening sky but this month it's eventually lost to the bright twilight. From Northern Hemisphere latitudes the planet can be glimpsed just after sunset towards the west during the first half of April. With an apparent magnitude of +1.4 it's looks like an unremarkable first magnitude "star". On April 22nd, Mercury (mag. -1.4) passes just over a degree north of Mars (mag. +1.4).
From southern latitudes, Mars is to all intent and purpose only visible during the first few evenings of the month.
Jupiter is now two months past opposition and despite fading in brightness it remains a brilliant evening object. As soon as darkness falls, the Solar System's largest and dominant planet is easily recognisable as a bright beacon of light amongst the faint stars of Cancer. From latitudes of Northern Europe and America, the giant planet sets after midnight by months end although the period of visibility is much reduced for those located further south.
Jupiter begins the month moving retrograde until April 8th when it reaches its second and final stationary point for 2015. After this direct motion (eastward) is again resumed with the event widely regarded as signalling the end of the opposition period. To the unaided eye, Jupiter will essentially appear stationary during April. However during the upcoming months direct motion will be obvious as it finally leaves Cancer and marches on towards Leo and Regulus.
On April 1st, Jupiter shines at magnitude -2.3 with an apparent diameter of 41 arc minutes. At the end of April the brightness has decreased to magnitude -2.1 and the apparent diameter to 38 arc minutes.
The waxing gibbous Moon (57% illuminated) passes 6 degrees south of Jupiter on April 26th.
Saturn continues to move slowly retrograde in northern Scorpius. The stunning "Ringed Planet" is located near the stars of the Scorpion "Sting" as it heads towards next month opposition. Saturn rises during the evening and remains visible for the rest of the night although it can be seen for considerably longer from equatorial and Southern Hemisphere latitudes. The planet also appears higher in the sky from such locations.
To the naked eye Saturn appears creamy in colour. It makes a nice contrast compared to the orange/red hue of first magnitude red giant Antares (α Sco mag. +1.0) located 8 degrees to the southeast. Through a telescope the planets rings are a beautiful sight. They are visible with just a small instrument and currently tilted so that they are wide open (tilted at 24 degrees). A medium size telescope of 150mm (6-inch) or 200mm (8-inch) aperture will show a wealth of details including subtle planet formations, divisions in the rings as well as up to half a dozen of Saturn's satellites.
During the month, Saturn brightens slightly from magnitude +0.3 to +0.1 with it apparent size increasing marginally from 17.8 to 18.4 arc seconds. On April 8th, the waning gibbous Moon passes 2 degrees north of Saturn.
Uranus reaches solar conjunction on April 6th and is therefore unsuitably placed for observation throughout April.
Neptune, mag. +8.0, reached solar conjunction at the end of February but remains unsuitably placed for observation from northern temperate latitudes during April. However, it can be seen in the early morning sky towards the east from equatorial and southern regions. By months end it rises up to four hours before the Sun from such locations.
The outermost planet of all is currently located in the faint constellation of Aquarius. Neptune never comes close to naked eye brightness though it can be seen with binoculars or small telescopes. On April 14th, large asteroid Vesta passes 2.7 degrees south of Neptune. Both objects are of the same magnitude and wide field scopes should easily show them in the same field of view.
The following day, the waning crescent Moon passes 3.6 degrees north of Neptune.