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. 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 constellation's next brightest star, Izar (ε Boo), shines at magnitude +2.35 and is much fainter than Arcturus. As a bright night time star, Arcturus has been significant to observers since antiquity. It was mentioned in the Bible and featured on old Chinese maps with the name 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.

Arcturus (credit:- Fred Espenak)

There are at least 6,000 stars bright enough to be visible with the naked eye. Under dark skies, up to about 3,000 can be seen at any one time (since one half of the Earth is in daylight). However, none can rival the brilliance of Sirius, the brightest night time star.

Located in the constellation of Canis Major, Sirius shines with an apparent magnitude of -1.46. It's noticeably 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". When close to the horizon it twinkles. Of course, the colour variations are only a result of the Earth's unsteady atmosphere. In practice, all stars do twinkle and to a lesser extent the planets as well.

Sirius (credit:- Fred Espenak)

The Delta Aquarids or Delta Aquariids is an annual meteor shower that takes place during July and August. It consists of two branches, the Northern Delta Aquariids and the Southern Delta Aquariids, of which the Southern Delta Aquariids is the stronger. This year's event takes place between July 12th and August 23rd, with peak activity occurring on July 29th when up to 16 meteors per hour can be expected (Zenithal Hourly Rate). In addition, the waxing crescent Moon sets around midnight local time leaving the pre-dawn hours free from its glare. This meteor shower is best seen from tropical and Southern Hemisphere locations.

The Northern Delta Aquariids stream is feeble in comparison. It peaks on August 7th with a maximum ZHR of 4.

Comet 96P/Machholz from HI-2 camera of STEREO-A spacecraft (credit:- NASA/ Johns Hopkins University)

Capella is a bright yellow star located in the northern section of sky. With an apparent magnitude of +0.08, it's marginally fainter than another bright northern star, Vega. Capella is the standout star of the relatively large constellation of Auriga, the Charioteer. The Milky Way passes through the heart of Auriga and as a result it contains numerous bright open clusters, nebulae and interesting stars.

Capella is the northernmost first magnitude star and is circumpolar from latitudes greater than +44 degrees. The star can still be spotted from most southerly regions including Australia, New Zealand and Argentina. However, it's not visible from the Falkland Islands or Antarctica.

Capella (credit:- Fred Espenak)

Almach (γ And) is one of the finest colour contrast double stars in the sky. To the naked eye, it appears as a single bright star of magnitude +2.10, but small scopes reveal an outstanding double, made up of a bright yellow or slightly orange primary and a fainter deep blue secondary. It's widely regarded as the second best colour contrast double in the sky, surpassed only by Albireo in Cygnus.

Almach (credit:- Mount Lemmon Observatory)

NGC 185, also known as Caldwell 18, is a dwarf spheroidal galaxy located in the southern part of Cassiopeia. It's about 2 Million light-years distant and like neighbouring dwarf galaxy NGC 147 is a satellite of the Andromeda galaxy (M31) and therefore a member of the Local Group. NGC 185 shines at mag. +9.1.

Dwarf spheroidal galaxies are similar to dwarf elliptical galaxies except they are spheroidal in shape, generally of a lower luminosity and recognised only as Local Group satellite galaxies. They are also believed to contain large amounts of dark matter.

NGC 185 (credit - James Gregory Telescope, St. Andrews, Scotland)

NGC 147, also known as Caldwell 17, is a dwarf spheroidal galaxy about 2.5 Million light-years away in the constellation of Cassiopeia. Together with neighbouring galaxy, NGC 185, it can be found in the southern part of the constellation, close to the Andromeda boundary. Both galaxies are satellites of M31 (Andromeda galaxy) and therefore members of the Local Group. They are best seen from Northern Hemisphere latitudes during the months of October, November and December.

Dwarf spheroidal galaxies are similar to dwarf elliptical galaxies except they are spheroidal in shape, generally of a lower luminosity and recognised only as Local Group satellite galaxies. They are also believed to contain large amounts of dark matter.

NGC 147 (credit:- Two Micron All Sky Survey - 2MASS)

NGC 40, mag. +10.7, is a planetary nebula located in the northern constellation of Cepheus. It was discovered by William Herschel on November 25, 1788. He described it as "a 9th magnitude star, surrounded with milky nebulosity". Herschel used his 475mm (18.7-inch) telescope to make the discovery, but today's amateur astronomers don't require such a large instrument and it can be glimpsed with just a 100mm (4-inch) refractor. 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 is therefore circumpolar from most northern latitudes. It's one of the finest examples of its type in the far northern section of sky. The best time to look for the nebula is during October, November and December when it appears high in the sky during early evening. The Bow Tie nebula is also visible, although lower down, from most tropical latitudes. However, from southern temperate locations it never rises above the horizon.

Finding NGC 40 can be tricky as it's positioned in a star poor region of eastern Cepheus. One method is to imagine a line connecting Errai (γ Cep - mag. +3.21) with γ Cassiopeiae (mag. +2.15). The planetary lies approximately one-third of the way along this line.

M105, mag. +9.8, is an elliptical galaxy visible with small telescopes that's located in the constellation of Leo. It was discovered by Pierre Méchain on March 24, 1781, which was three days before he discovered M101. However, due to unknown reasons, this galaxy wasn't included in Charles Messier's final published catalogue version. It was eventually added by Helen Sawyer Hogg in 1947, together with M106 and M107. William Herschel independently rediscovered M105 on March 11, 1784.

M105 is the brightest elliptical member of the Leo I or M96 group of galaxies that also includes M95, M96 and at least another 21 fainter members. This grouping is one of many that lie within the Virgo Supercluster. M105 is located 35 million light-years distant and is known to contain a supermassive black hole at its centre.

They are best seen during the months of March, April and May.

M105 Elliptical galaxy by the Hubble Space Telescope (credit:- NASA, The Hubble Heritage Team (AURA/STScI))

M102 is a galaxy catalogued by Charles Messier that hasn't yet been explicitly identified. It was Pierre Méchain who made the original observation in March 1781, before passing the information onto Messier who catalogued it without verification. However, Méchain believed M102 was an error and even wrote a letter on May 6, 1783 expressing his view that the object was in fact a duplicate of M101. Historical, evidence based on Messier's description of the galaxy, combined with its co-ordinates, suggest that M102 could well be lenticular galaxy NGC 5866, also known as the Spindle galaxy. A number of other possible candidates have emerged, but it seems likely that Messier and Méchain both at some time observed NGC 5866, and therefore we include it as the missing item.

The Spindle galaxy (mag. +9.9) is located at the southern edge of the far northern constellation of Draco. It's positioned four degrees southwest of Iota Draconis (ι Dra - mag. +3.3). Directly west of NGC 5866 are the seven stars that form the famous "Plough" or "Big Dipper" asterism of Ursa Major.

NGC 5866 is a challenging binocular object, but much easier to spot with small scopes. It's best seen from Northern Hemisphere locations during the months of April, May and June. From latitudes of 35N or greater, this galaxy is circumpolar and therefore never sets.

M91, mag. +10.3, is a barred spiral galaxy located in the southern part of the constellation of Coma Berenices. It's a member of the Virgo cluster and was discovered by Charles Messier on March 18, 1781. This was a most productive night for Messier. He discovered eight objects, all of them Virgo cluster galaxies, and also rediscovered globular cluster M92 in Hercules. When recording the position of M91, Messier incorrectly referenced its location from M58 instead of intended galaxy M89. It was only a degree mistake, however the result meant that the object was missing for almost 200 years! Some astronomy detective work by William Williams finally solved the mystery. In 1969, he pinpointed its location and concluded that M91 was almost certainly the same galaxy as NGC 4548.

Finding M91 can be challenging. The galaxy is positioned about a degree north of the Coma Berenices-Virgo constellation boundary but there are no bright stars in the vicinity. The general area of sky can be located by imagining a line connecting Denebola (β Leo - mag. +2.1) with Vindemiatrix (ε Vir - mag. +2.8). About 60% of the way along this line is elliptical galaxy, M89 (mag. +10.0), with M91 positioned a further two degrees directly north.

M91 and the Virgo cluster galaxies are best seen during the months of March, April and May.

Mercury

Mercury reaches greatest elongation east on July 30th, when it moves out to 27.2 degrees from the Sun. From Southern Hemisphere locations, the planet can be seen early in the month low down towards the northwestern horizon just after sunset. Visibility then improves gradually each evening until greatest elongation is reached. For example, from latitude 35S (approx. equal to Sydney, Cape Town and Santiago), Mercury will be 16 degrees above the horizon, 45 minutes after sunset. From the middle to the end of the month, its magnitude decreases from -0.5 to +0.3.

On July 25th, the thin crescent Moon passes less than a degree north of Mercury and on the following day Mercury passes a degree south of Regulus (mag. +1.4).

From mid-latitude northern temperate locations the planet isn't so well placed, but can be spotted hugging the western horizon near the end of the month.

Mercury 45 minutes after sunset from mid-latitude southern locations on July 30, 2017 (credit:- stellarium/freestarcharts)

Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at mag. +2.1 but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.

Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.

Algol System (credit:- Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

M107 is a loose globular cluster of eighth magnitude located in Ophiuchus. It's a difficult binocular object that's much easier to spot with small telescopes. This globular was discovered by Pierre Méchain in April, 1782 and independently re-discovered by William Herschel on May 12, 1793. Herschel was also the first person to resolve it into stars. The cluster is one of the additional items that were not included in Messier's final catalogue version, but added much later by Helen Sawyer Hogg in 1947. She also added M105 and M106.

M107 is located 20,900 light-years from Earth and spans 13 arc minutes of apparent sky, which corresponds to a spatial diameter of 80 light-years. It contains 100,000 stars and has an estimated age of 13.95 billion years, making it one of the oldest known globulars.

M107 globular cluster by the Hubble Space Telescope (credit:- NASA, The Hubble Heritage Team (AURA/STScI))

Saturn is now at its best for the year and on June 15th opposition is reached. Currently located in Ophiuchus the planet is visible all night long, but with a declination of -22 degrees it's much better placed from southern and tropical locations. For example, Saturn reaches a maximum altitude of 78 degrees and is visible for over 13 hours from Sydney, Australia. Whereas from New York City, it climbs just 28 degrees high with a visibility period lasting 9 hours.

Saturn imaged by the Hubble Space Telescope (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

M94 is a nice spiral galaxy located in the constellation of Canes Venatici. Due to its eye-like resemblance, it's also known as the Cats Eye galaxy or Crocs Eye galaxy. With an apparent magnitude of +8.5, this galaxy is a difficult binocular object that requires dark skies and good transparency. At best through such instruments, it appears as only a small faint hazy patch of light.

M94 is one of the nearest galaxies beyond our Local Group. It's located about 16 million light-years distance and belongs to the M94 Group, a collection of between 16 and 24 galaxies within the Virgo Supercluster. Pierre Méchain discovered it on March 22, 1781 and it was subsequently confirmed and catalogued two days later by Charles Messier.

The constellation of Canes Venatici is faint but the brightest star Cor Caroli (α CVn - mag. +2.9) can be quite easily found since it's due south of the Plough or Big Dipper asterism of Ursa Major. The second brightest star in the constellation is Chara (β CVn - mag. +4.2), which is positioned just over 5 degrees northwest of Cor Caroli. M94 is 3 degrees east and a fraction south of Chara. Further eastwards is the fine Sunflower Galaxy (M63).

Mercury

Mercury remains an early morning object from southern and tropical regions during the first half of the month. From such locations, the planet can be glimpsed low down above the east-northeastern horizon just before sunrise. However, it's not long before it's lost to the bright twilight sky and by the second week of the month it will be difficult to spot. Mercury then heads towards superior conjunction, which it reaches on June 21st.

From northern temperate latitudes, the planet is unsuitably placed for observation throughout June.

Mercury 45 minutes before sunrise from mid-latitude southern locations (credit:- freestarcharts)

M92, mag. +6.4, is a bright globular cluster located in the northern part of the constellation of Hercules. Despite being almost visible to the naked eye, it's often overlooked due to its close proximity to more spectacular, M13.

M92 is one of the original discoveries of Johann Elert Bode who found it on December 27, 1777. He described it as "a nebula that's more or less round with a pale glow". Charles Messier independently rediscovered it and catalogued it on March 18, 1781. Incidentally, this proved to be Messier's most productive night, during which he discovered another 8 objects all of them Virgo Cluster galaxies (M84 to M91). As with many globulars, it was William Herschel who first resolved it into stars. To date, only about 16 variables have been discovered in M92 of which 14 are of the RR Lyrae type, and one is a rare globular eclipsing W Ursae Majoris type binary.

M92 Globular Cluster (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

M87 is a supergiant elliptical galaxy that's a prominent member of the Virgo cluster of galaxies. It's one of the largest and most luminous galaxies known and a strong source of electromagnetic radiation, particularly radio and X-ray emissions. At the centre of M87 is a supermassive black hole from which a jet of extremely energetic plasma extends outwards for at least 5,000 light-years. The galaxy is therefore an interesting object for both professional and amateur astronomers alike.

With an apparent magnitude of +8.6, M87 is the second brightest of the Virgo cluster galaxies. Only M49, at mag. +8.4, is marginally brighter. On dark moonless nights, M87 is visible with 7x50 or 10x50 binoculars, appearing as a faint hazy patch of light. This galaxy was one of eight discovered by Charles Messier on March 18, 1781. He also re-discovered fine globular cluster, M92, on the same day.

M87 lies at the heart of the Virgo cluster. It can be found by imagining a line connecting Denebola (β Leo - mag. +2.1) with Vindemiatrix (ε Vir - mag. +2.8). Just over half way along this line is M87. Faint elliptical galaxy M89 is positioned just over a degree east of M87 with the galaxy pair M84/M86 located 1.5 degrees to the northwest.

The Virgo galaxies are best seen during the months of March, April and May.

Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at mag. +2.1 but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.

Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.

Algol System (credit:- Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

M97, also known as the Owl Nebula, is a famous planetary nebula located in the constellation of Ursa Major. It was discovered by Pierre Mechain on February 16, 1781 and is one of only four planetary nebulae listed in the Messier catalogue. Although not particularly bright at magnitude +9.9, it's a superb object and regarded as one of the most complex examples of its type. The name Owl Nebula was first coined in 1848 by William Parsons the 3rd Earl of Rosse, who noticed owl-like "eyes".

Locating M97 is easy as it's positioned only 2.5 degrees southeast of bright star Merak (β UMa - mag. +2.3). This is the southwest corner star of the bowl of the famous Plough or Big Dipper asterism of Ursa Major. In the same wide field telescope field of view, 50 arc minutes northwest of M97, is barred spiral galaxy M108 (mag. +10.2).

The Owl Nebula is best seen from Northern Hemisphere latitudes during the months of March, April and May. From latitudes north of 35N, it's circumpolar and therefore never sets.

M83, also known as the Southern Pinwheel Galaxy, is a barred spiral galaxy approximately 14.7 million light-years distant in the eastern section of Hydra. It's one of the closest barred spirals, a showpiece galaxy and the finest example of its type in the sky. With an apparent magnitude of +7.5, this galaxy is visible with 7x50 or 10x50 binoculars under dark skies as a patch of light with a bright centre.

At a declination of 30 degrees south, M83 is best seen from Southern Hemisphere or equatorial regions during the months of April, May and June. It's the southernmost galaxy in the Messier list and therefore a difficult object from mid-northern latitudes, that never climbs particularly high above the horizon.

Although a relatively bright galaxy, M83 can be tricky to find since it's located in an area of sky devoid of bright stars. It's positioned 6 degrees south of the mid-point of an imaginary line connecting γ Hydrae (mag. +3.0) with π Hydrae (mag. +3.3). M83 was discovered by Nicholas Louis de Lacaille at the Cape of Good Hope in South Africa on February 23, 1752 and was added by Charles Messier to his catalogue on February 17, 1781.

M103 is a small loose but sparkling open cluster of at least 40 stars located among the Milky Way star fields of Cassiopeia. At magnitude +7.4, it's beyond naked eye visibility but an easy binocular target.

M103 was discovered by Pierre Méchain in either March or April of 1781. He reported his discovery to Charles Messier, who would normally take the opportunity to observe the object himself, but this time he didn't probably due to lack of time or unfavourable weather conditions. This cluster along with M101 and M102 were the last objects added by Messier. Items M104 to M110 were included much later during the 20th century.

Messier 103 Open Cluster (credit:- NOAO/AURA/NSF)

M51, also known as the Whirlpool Galaxy, is a grand design spiral galaxy located in the constellation of Canes Venatici. It's one of the most famous galaxies in the sky, appearing face-on when viewed from Earth. At magnitude +8.4, it's relatively bright and visible in binoculars, especially from dark sites. M51 has a much smaller dwarf companion, known as NGC 5195, and together they form the finest and most studied example of an interacting galaxy pair in the sky.

M51 was one of Charles Messier original discoveries on October 13, 1773. His friend Pierre Méchain discovered NGC 5195 on March 20, 1781. Messier described M51, as a faint nebula without stars, that was difficult to see. In his catalogue of 1781, he put M51 and NGC 5195 in the same note and therefore some confusion exists. Was he referring to M51 as the larger galaxy, or did he mean the pair itself. If it was the pair, then the main galaxy should really be referred to as M51A, with NGC 5195 separately known as M51B.

M81, also known as Bode's galaxy, is a large bright spiral galaxy located 11.8 million light-years away in the constellation of Ursa Major. With an apparent magnitude of +6.9, it's easily visible with binoculars and is a superb target for all sizes of telescopes. This galaxy is a striking example of a grand design spiral and exhibits near perfect and well defined spiral arms.

In the same binocular and low magnification telescope field of view as M81 is another prominent galaxy, M82. At mag. +8.4, M82 is fainter than M81 and a very different type of galaxy. It's a starburst galaxy, in which stars are been formed at exceptionally high rates. Also known as the Cigar galaxy, M82 is the prototype object and provides a striking contrast to the near perfect spiral shape of M81. Together, they form a popular visual and imaging target for amateur astronomers.

Both M81 and M82 were discovered by Johann Elert Bode on December 31, 1774. Pierre Mechain independently rediscovered them in August 1779. He reported his observations to Charles Messier who added them both to his catalogue on February 9, 1781.

Finding M81 is not particularly difficult as the famous Plough asterism of Ursa Major can be used as a starting point. First focus on Dubhe (α UMa - mag. +1.8), the northwest corner star of the bowl. The M81 / M82 pair is located 10 degrees to the northwest of this star with M82 positioned 38 arc minutes directly north of M81.

Mercury reaches greatest western elongation on May 17th, when it will be positioned 25.8 degrees from the Sun. This apparition offers an excellent opportunity for observers at southern and tropical latitudes to spot the elusive planet. From such locations, Mercury can be seen low down above the east-northeastern horizon just before sunrise throughout May and until the second week of June. However, from northern temperate locations the planet is not well placed, although from some locations it can be spotted around the time of greatest elongation but always very close to the horizon.

Mercury as imaged by the MESSENGER spacecraft (credit - NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Mercury

Mercury reaches greatest elongation west on May 17th when it's positioned 26 degrees from the Sun. The planet is not well placed for observation at northern temperate locations, but from southern latitudes this happens to be the most favourable morning apparition of the year. With an extended period of visibility it can be seen in the morning skies until mid June.

On May 1st, from latitude 35S (approx. equal to Sydney, Cape Town and Santiago), Mercury rises over an hour before the Sun. At magnitude +2.5, binoculars will be required to spot the elusive planet against the bright dawn sky. Positioned a degree from Mercury is fainter Uranus (mag. +5.9). As the month progress the visibility of Mercury improves until greatest elongation west is reached. On May 17th, the planet shines at magnitude +0.5 and appears 15 degrees above the east-northeastern horizon, 45 minutes before sunrise. For the remainder of the month it gradually sinks back towards the horizon. It should be noted that Mercury improves in brightness throughout May and on the 31st it will shine at magnitude -0.3. Brilliant Venus is positioned above Mercury throughout this time.

From mid-northern temperate latitudes, Mercury can be seen for a short while around the time of greatest elongation. However, it's always low down and at best just a few degrees above the horizon.

On May 24th, the thin waning crescent Moon passes 2 degrees south of Mercury.

The annual Eta Aquariids meteor shower, occasionally written as Eta Aquarids, peaks on May 5th / 6th and this year's event is favourable as the waxing gibbous Moon will have set a few hours before sunrise. The shower is best seen in the early morning hours just before sunrise from southern and equatorial regions.

Parent Comet

The parent body for the Eta Aquariids meteor shower is Halley's comet (1P/Halley). This isn't the only annual shower associated with the famous comet, the Orionids in October also originate from the same source. Although Halley has now left the inner Solar System and won't return until 2061, it's worth remembering that every Eta Aquariids meteor is a small part of the famous comet burning up in the Earth's atmosphere.

Halley's Comet (credit:- ESA)

Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at mag. +2.1 but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.

Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.

Algol System (credit:- Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

The annual Lyrids meteor shower peaks during the night of April 21st/22nd and this year's event promises to be a good one, as the 23% illuminated waning crescent Moon in Aquarius, won't significantly interfere. Normally, you can expect to see up to 20 meteors per hour under ideal conditions. In addition, the Lyrids do occasionally produce brilliant fireballs, that streak through the sky and cast shadows as they disintegrate in the Earth's atmosphere.

View just after midnight on April 22, 2017 from northern temperate latitudes (credit:- stellarium/freestarcharts)

Jupiter remains a dazzling object this month and on April 7th the planet is at its best for the year, when opposition is reached. On this day, it will shine at mag. -2.5 and to the naked eye, the largest planet in the Solar System is unmistakable and easily brighter than any night-time star. It's currently located in Virgo, 4 degrees northeast of the constellation's brightest star, Spica (α Vir - mag. +1.0).

At opposition, Jupiter is visible all night long. The planet rises in the east at sunset, reaches its highest point in the sky around midnight before setting in the west at sunrise. It's best seen from equatorial regions, although from northern and southern temperate locations it still reaches a reasonable altitude during late evening and early morning.

Jupiter and Ganymede as seen by Hubble Space Telescope on April 9, 2007 (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

Mercury

Mercury reaches greatest eastern elongation (GEE) on April 1st, when it moves out to 19 degrees from the Sun. On this day from mid-latitude northern locations, the innermost planet will shine at mag. -0.2, 10 degrees above the western horizon, 45 minutes after sunset. It remains visible, although fading, until the second week of the month when eventually lost to the bright evening twilight. From mid-northern latitudes, this also happens to be the best evening apparition of the year. However, from southern locations, Mercury is unsuitably placed for observation during this time.

The diagram below shows the altitude of Mercury, 45 minutes after sunset as seen from latitude of 51.5N (e.g. London, England). A similar view exists at other northern temperate latitudes.

Mercury 45 minutes after sunset from mid-latitude northern locations (credit:- freestarcharts)

Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at mag. +2.1, but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.

Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.

Algol System (credit:- Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

Mercury, the innermost planet, is now heading towards greatest eastern elongation (GEE), which it reaches on April 1, 2017. From mid-March, observers at northern-based locations should be able to spot the elusive planet, low down above the western horizon just after sunset. Each subsequent evening it improves in visibility until GEE is reached. After that, Mercury sinks gradually back towards the horizon until about 10 days later when it becomes lost to the bright twilight sky. This also happens to be the best evening apparition of the year from northern locations.

From southern latitudes, Mercury is unsuitably placed for observation this time.

Mercury as seen by the MESSENGER space probe (credit:- freestarcharts)

NGC 6302, also known as the Bug Nebula or Butterfly Nebula, is a bipolar planetary nebula in the constellation of Scorpius. It's an unusual object, complex in structure with an incredibly hot star at its core. At a temperature of 250,000 degrees Celsius, this dying star is one of the hottest known in the galaxy. However, since it radiates predominantly in UV and is shrouded by dust, it's visually challenging to observe.

The discoverer of the Bug Nebula is debatable. Edward E. Barnard observed the planetary in 1880, although some references suggest that James Dunlop may have found it in 1826. The nebula itself shines at mag. of +9.6 and therefore well within the reach of medium size backyard scopes.

NGC 4559 is a spiral galaxy located in the constellation Coma Berenices. At mag. +10.0 it can be spotted with binoculars, but is challenging, requiring dark skies and patience. However, its high surface brightness does somewhat help. The galaxy is a nice telescope target that offers something for all sizes of backyard instruments.

NGC 4559 was discovered by William Herschel on April 11, 1785 and is estimated to be 30 million light-years distant. It's best seen from northern locations during the months of March, April and May.

NGC 4559 (credit:- NASA, ESA, and The Hubble Heritage Team (STScI/AURA))

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
Evening
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
Midnight
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Morning
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
Evening
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
Midnight
West:- Jupiter
North:- Saturn
East:- Neptune
Morning
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

Guides / Star Charts

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