Mars presents a superb UK observing opportunity in 2020 as the planet reaches opposition at a decent altitude in mid-October. Mars will also be in the middle of a ‘retrograde loop’ in the sky, and its apparent motion against the background stars will appear to reverse.
So far during 2020, its apparent motion against the stars has been eastward, but this will slow to a stop on 10 September, when Mars reaches a ‘stationary point’.
After this, it tracks west and continues in this direction until another stationary point is reached on 16 November, when Mars resumes moving east.
The illusion is caused by the relative position and speed of Mars in its orbit compared to Earth: our faster orbit allows us to overtake Mars, and this creates the looped path in the sky.
All the planets appear to perform retrograde loops, decreasing in apparent size with distance from Earth. And this is the key point: it is Mars’s close proximity that makes its opposition and retrograde loop really impressive.
You can reveal the loop by photographing Mars against background stars with a mid- to wide-angle lens on a camera.
Take a photograph on every clear night through to the end of the year, load each into a layer-based editor like GIMP and align using the stars. Set the blend mode of all upper layers to lighten and Mars will show through, revealing the loop.
How to photograph the Mars retrograde loop
The term planet comes from the Ancient Greek meaning ‘wandering star’. They’re called this simply because, looking to the naked eye like stars, they appear to wander against the background of fixed constellations.
It’s possible to show this fascinating motion against the background stars using a camera. The basic idea isn’t that complicated.
The stars will be your reference markers and, as long as you have three or more common stars recorded across shots, it will be possible to align all your images to show how Mars is moving between them.
Once you get going, it’s rather exciting to collect yet another Mars positional image to add to the set. Add them all together as described below and you’ll have an amazing record of the temporarily backward motion of Mars in the sky.
For this project you will need:
- DSLR camera
- Fixed tripod
- 50mm or shorter focal length lens
- Remote shutter release
- Layer-based graphics editor
The positional shot can be made with a high ISO (800–1600), low lens f/number and a fixed tripod. The shorter the focal length, the longer the exposure you can take before star trailing appears. Use an equatorial tracking mount for deeper shots to keep the stars sharp.
Focus the lens as accurately as possible. If the Moon is up, turn the lens focusing to auto-focus, point the camera at the Moon and half-press the shutter button. Once focused, set the lens to manual focus. If you have Live View focusing, use that directly on Mars.
Using the same focal length lens, take exposures that record Mars and at least three stars. Start with around 15 seconds. A remote shutter release avoids camera shake. Sky brightness becomes an issue from May, so use shorter exposures to counteract this.
Load all images into a layer-based editor on separate layers. Using the bottom image as a reference, align the stars in each frame. Make the upper image temporarily transparent to do this. Then turn the blend mode of each upper layer to lighten to reveal the retrograde loop.
Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night. This article originally appeared in the September 2020 issue of BBC Sky at Night Magazine.