Guides / Star Charts

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Mercury

Mercury is a difficult target for observation this month. The innermost planet can be seen low down in eastern twilight during the first week of September, but at best rises only about 45 minutes before the Sun.

Mercury reaches perihelion on September 2nd, when it's positioned 0.307 AU (approx. 45.9 million kilometres or 28.5 million miles) from the Sun. On September 5th, the planet (mag. -1.1) passes a degree north of Regulus (α Leo - mag. +1.4). It then moves towards superior conjunction, which it reaches on September 20th.

Venus

Venus remains a brilliant early evening object from southern and tropical locations throughout September. The unmistakable planet is located in Virgo, and appears as a beacon of light above the western horizon just after sunset. Venus increases in magnitude from -4.6 to -4.8, with its illuminated phase decreasing from 40% to 18%. As the month progresses the planet's apparent diameter increases from 29 to 46 arc seconds.

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 brightness change. 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 Dumbbell Nebula or M27 is a showpiece object that's a popular visual and imaging target for amateur astronomers. It's arguably the finest planetary nebula in the night sky and was the first of its type to be discovered. The name derives from its resemblance to a dumbbell shape, but it has also been compared to an apple core and an hourglass figure. With a mag. of +7.4, M27 is the second brightest planetary nebula in the sky. Only the Helix Nebula (NGC 7293) in Aquarius is brighter. However, the Dumbbell Nebula has a higher surface brightness and therefore the easier of the two targets to spot.

M27 The Dumbbell Nebula (credit:- ESO)

M57, the Ring Nebula, is a showpiece planetary nebula located in the constellation of Lyra. It's probably the most well-known, studied and photographed object of its kind and a perennial favourite with amateur astronomers. The nebula is relatively bright at magnitude +8.8 and easy to locate. It can be found about 40% the way along an imaginary line connecting stars, Sheliak (β Lyr - mag. +3.5) and Sulafat (γ Lyr - mag. +3.2). For Northern Hemisphere observers, it appears high in the sky during the warm summer months although from southern latitudes it appears much lower down.

M57 was discovered by Antoine Darquier de Pellepoix in January 1779.

M57 The Ring Nebula (NASA, ESA, and The Hubble Heritage Team (credit:- STScI/AURA))

The Perseid meteor shower is among the finest annual meteor showers, always reliable, producing a flux of fast and bright meteors. This year's event peaks on the nights of August 11-12 and August 12-13. The best time to look is during the early morning hours, and on this occasion it's extremely favourable as the new Moon will not interfere. At peak of about 60 meteors per hour are predicted. Recent analysis by NASA has rated the Perseids as the best meteor shower when it comes to fireballs.

The Perseids favour northern based observers. For those at southern locations, the radiant remains close to or never rises above the horizon. This considerably reduces the amount of visible meteors, although it's possible to spot some of them coming up above the northern horizon.

A Perseid flashes through the sky (credit - Andreas Möller via wikimedia.org)

M16, also known as the Eagle Nebula, is a young open cluster of stars embedded within an extremely large cloud of interstellar gas and dust in the constellation of Serpens (Cauda). It's located in the next inner spiral arm of the Milky Way, 7,000 light-years distant. The emission part of the nebula or HII region is catalogued as IC 4703 and is an active star-forming region, which has already created a significant cluster of young stars. The cluster itself lies at the heart of the Eagle Nebula and is known as NGC 6611. M16 was discovered by Philippe Loys de Chéseaux in 1745-6, but Charles Messier was the first to record the associated nebulosity on June 3, 1764.

The constellation of Serpens is faint but unique as its split into two separate sections. One half, named Serpens Caput, lies to the west of Ophiuchus and the other half, Serpens Cauda, lies on the eastern side of Ophiuchus. At the very southern tip of Serpens Cauda close to the Scutum and Sagittarius border is M16. It can be found 2.5 degrees west of γ Sct (mag. +4.7) and a few degrees north of the Omega Nebula (M17), M18 and the Sagittarius Star Cloud (M24). This beautifully rich area of the sky is a delight to scan with binoculars.

The Eagle Nebula was immortalised in 1995 when imaged several times by the Hubble Space Telescope. The resulting iconic photograph, titled the Pillars of Creation, showed three magnificent columns of interstellar gas and dust displayed in sensational detail.

M16 The Eagle Nebula showing the Pillars of Creation star forming region (credit:- NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University))

Mercury

Mercury passes through inferior conjunction on August 8th. The fast moving planet then moves into the morning sky, reaching greatest elongation west on August 26th. On this day, it will shine at magnitude -0.1 and be positioned 18 degrees from the Sun. From northern temperate latitudes, Mercury will rise 90 minutes before sunrise and be visible very low down above the eastern horizon.

From southern locations, Mercury is not suitably placed for observation this month.

Venus

Venus remains a brilliant early evening object, visible above the western horizon as soon as it's dark enough. On the first day of August, the planet moves from Leo into Virgo where it remains for the rest of the month.

Venus increases in magnitude -4.3 to -4.6, with its illuminated phase decreasing from 57% to 41%. During this time, the planet's apparent diameter increases from 20 to 29 arc seconds. On August 14th, the waxing crescent Moon passes 6 degrees north of Venus offering pleasant early evening viewing. On the last day of the month, Venus passes within 2 degrees of first magnitude Spica (α Vir - mag. 1.0), the brightest star in Virgo.

Moon, Venus and Jupiter in the evening sky from northern temperate locations just after sunset on August 14, 2018 (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 brightness change. 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)

Algol is located in Perseus among the stars of the northern Milky Way. It's positioned west of mag. +0.1 Capella (α Aur) and southeast of the well known "W" of Cassiopeia. The finder chart below shows the position of Algol along with magnitude data of some surrounding stars for comparative purposes.

Finder Chart for Algol (credit:- freestarcharts)

Finder Chart for Algol - pdf format (credit:- freestarcharts)

July 2018 is a special month for Mars observers with the planet reaching opposition on July 27th, and making its closest approach to Earth since 2003. On this occasion the red planet will peak at magnitude -2.8 and come within 0.386 AU (approx. 57.7 million kilometres or 35.9 million miles) of the Earth. Of all planets, only Venus currently appears brighter than Mars. On the same day as Mars reaches opposition, a total lunar eclipse can be seen from Africa, Asia, South America, Europe and Australia.

Mars is currently moving retrograde in the faint constellation of Capricornus. To the naked eye it's unmistakable, deep orange in colour and easily brighter than any surrounding stars. Located about 30 degrees west of Mars is fainter Saturn (mag. +0.2). At opposition, Mars is visible all night, but due to its southerly declination, it's much better placed from Southern Hemisphere and tropical latitudes. For example on July 27th from London, Mars rises at 21:35 and sets at 04:45; a visibility period of just over 7 hours. Whereas from Sydney, it rises at 16:50, sets at 07:25 and therefore is visible for over 14 hours. In addition, Mars reaches a maximum altitude of just 13 degrees above the southern horizon from London. For comparison, from Sydney it can be seen 81 degrees high and therefore appears almost overhead.

Hubble Space Telescope image of Mars on August 26, 2003 (credit:- NASA/J. Bell/M. Wolff)

The orbit of Mars is eccentric which means the difference in distance between its closest (perihelion) and furthest (aphelion) orbital nodes are considerable. At perihelion, Mars is 1.3815 AU (206.7 million kilometres or 128.4 million miles) from the Sun, whereas at aphelion it's 1.666 AU (249.2 million kilometres or 154.9 million miles) away. This corresponds to an orbital eccentricity of 9.3%, compared to just 1.7% for the Earth. As a result, the brightness and apparent size of Mars, as viewed from Earth, varies greatly depending on the positions of the two planets in their respective orbits.

Another reason why Martian oppositions are special is because they don't occur as frequently as other outer planets. Generally speaking, Jupiter, Saturn, Uranus and Neptune reach opposition once a year, but Mars takes roughly 780 days from one opposition to the next. The reason is due to orbital dynamics. Mars is the nearest outer planet, orbits faster compared to the others and therefore it takes the Earth twice as long to catch up.

The position of Mars at opposition on July 27, 2018 (credit:- freestarcharts)

The position of Mars at opposition on July 27, 2018 - pdf format (credit:- freestarcharts)