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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)

At opposition, Mars spans 24.3 arc seconds of apparent sky and a small telescope under good seeing conditions will reveal surface markings. When viewed through an 80mm (3.1-inch) instrument, it's possible to spot the polar cap as well as major features such as the Syrtis Major on its salmon-pink surface. On initial observation the planet may appear bland, but with time and patience it's possible to tease out subtle details. Also don't be afraid to push up the magnification as far as seeing allows.

The co-ordinates of Mars at opposition are: R.A. = 20h 31m 47s and Declination = -25d 32m 36s (J2000)

Moon, Mars and Saturn as seen just before midnight on July 27, 2018 from London, England (credit:- stellarium/freestarcharts)