September 2020

by Roger Davies

A s we enter September, the days are rapidly diminishing in length, and the Sun moves into the southern hemisphere as Summer gives way to Autumn.

This month, Mars will brighten and execute a switchback in the sky as it is approaches opposition (opposite the Sun in the sky). We’ll lose the double act of Jupiter and Saturn that have been with us in the southern skies all summer, and we will get a chance to spot the nearest giant galaxy, the most distant object visible with the naked eye.

Early in the month, Mars rises just after sunset in the constellation of Pisces and will brighten through the month as it approaches opposition on 13th October. It has an unmistakeable orange-red hue. Mars is the most visited of the planets and already five space missions heading its way have been launched in 2020, two from the USA, two from China and one from the United Arab Emirates. Mars is much smaller than the Earth, just a little over a tenth of Earth’s mass, and has lost almost all of its atmosphere. Mars' surface is continually bombarded by radiation and the little water remaining is frozen below the surface. Since the Viking missions of the 1970s we've been aware that, in the distant past, flowing liquid carved river systems into the Martian terrain. Over the past year, by putting together images and high precision radars from orbiters, with the results from rovers such as NASA's Curiosity, we've deduced that it was water that created those river systems around 3.5 billion years ago. As we believe that liquid water is essential for life, is it possible that simple life forms evolved at that time that might still exist below the Martian surface today?

Valley networks, photographed in the 1970s by NASA's Viking missions, first suggested that rivers once flowed on Mars.

Perseverance, one of NASA's missions, will provide our first chance of a definitive answer. It will collect and seal rock samples in titanium tubes that a future ESA rover will collect and return to Earth in 2028. In the meantime ESA's Rosalind Franklin mission (scheduled launch 2023) will drill 2m down below the surface to reach material that has not been damaged by the radiation that impacts directly on Mars' surface. The in situ analysis of these deep samples could reveal the presence of organic molecules that may provide evidence of past life.

Watching Mars through the next few weeks you will see for yourself one of the puzzles that perplexed ancient astronomers. Mars normally moves eastward with respect to the 'fixed stars' but this month it starts a switchback. On 10th September, Mars changes direction and moves westward until 16th November when it reverses again and resumes its eastward motion (see figure below). The puzzle the classical astronomers faced was how to arrange this when they believed the planets orbited the Earth? The solution, recorded in Ptolemy's Almagest published in the second century AD, was to have Mars move on a small circle, an epicycle, while the centre of the epicyclic circle orbited Earth on a large circle. This was just one example, albeit the most famous, of the unnecessary complexity added to 'save the hypothesis' of the geocentric Solar System. All these complications were swept away in 1453 when Nicolaus Copernicus proposed the heliocentric model where all the planets orbit the Sun with the innermost moving quickest. In this way the retrograde motion of Mars can be explained as the Earth overtakes Mars when close to opposition.

Click to enlarge the source of this image
The position of Mars from May 2020 to March 2021 illustrating the retrograde motion from mid-September mid-through November.

The retrograde motion of Mars is an illusion created by the greater orbital speed of Earth that causes Mars to appear to move backwards when it is close to opposition. Source: Sky at Night Magazine, Credit: Pete Lawrence.

Early risers can see Venus before sunrise, it will be significantly brighter than Mars. In a dramatic juxtaposition on 14th September the waning crescent Moon is in conjunction with Venus — they are about 4°apart in the constellation of Cancer.

To help you find your way around the sky a good interactive sky map can be found here.

While you are out spotting Mars, you can try to find the nearest giant galaxy to us, the spiral nebula in Andromeda, which is similar in size and mass to our own Milky Way. At a distance of two million light years is the most distant object you can see without the aid of binoculars or a telescope. The chart shows you where to look, it is a faint fuzzy patch, not at all easy to see. To see it you need to be in a dark location away from street lights, only try on the nights around New Moon (a few days around 17th September and 16th October), and get fully dark adapted, so be outside in the dark for at least 15 minutes. The final trick is to use 'averted vision', don’t look directly in the direction of the object but a little way away so that the image falls on the periphery of your retina where there are more 'rods' that are sensitive to low light levels. If you do see it, reflect that the photons that hit your retina set off two million years ago, well before there was any human life on Earth!

The Andromeda nebula is between Andromeda and Cassiopeia. It is best found by starting from the corner of the square of Pegasus and moving along two stars into Andromeda. Source: here.

The Andromeda nebula is the nearest giant spiral galaxy. Credit ESA/Hubble images: Tony and Daphne Hallas.

Happy stargazing!

SEE ALSO by Roger Davies:
Stargazing • February 2021
Stargazing • November 2020
Stargazing • September 2020
Comet Neowise • July 2020
Stargazing • July & August 2020
Stargazing • June 2020
Stargazing • May 2020
Stargazing • April 2020

Roger Davies is a resident of the village of Kidlington, Oxfordshire.

Roger is the Philip Wetton Professor of Astrophysics and Director of the Hintze Centre for Astrophysical Surveys at the Department of Physics, University of Oxford,

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