Solar Eclipse by Phobos

NASA’s Perseverance Rover shot this video (actually a time lapse of multiple images) on Feb 8, 2024. It’s not the first one I’ve seen, and it’s not the first I’ve written about, but every eclipse is unique and worth taking a look. Eclipses on Mars are quite different than eclipses on Earth because of the size of the moons involved and their distances from the orbited body. Further, the Sun in smaller when seen from Mars, with an angular diameter of 0.35° compared to 0.50° when seen from Earth.

FYI: The dozen or so bright points of light in the video are not stars. They are hot pixels caused by defects in, or damage to, the CMOS imaging sensor. This can be  caused by cosmic rays and other sources of radiation.

You can see in the video that, even with the smaller Sun, Phobos is just too tiny to cover the Sun completely. Phobos is about 157 times smaller than our Moon and measures only 27×22×18 km. So technically, this is more properly referred to as a transit of Phobos and not an “eclipse.” This is what the shadow cone geometry looks like:

The top scenario cannot occur with Phobos. If the umbra reached to ground level, Perseverance would have seen a total solar eclipse with the entire Sun blacked out. What actually happens is the bottom scenario. The umbra never reaches the surface of Mars. And if you are located in the antumbra (as Perseverance was) you will see Phobos transit the Sun as it does in the video.

Note that in either case, an observer in the penumbra will see Phobos graze the Sun to varying extents in a partial transit — but it will never show its full silhouette.

During a transit, it wouldn’t get that dark on the ground (as it does on Earth during a total eclipse). Your eyes would detect a slight dimming in the brightness of the landscape, but only by around 18%. If you were wearing eclipse glasses, you’d easily spot Phobos crossing the disc of the Sun — it appears around half the size of our Moon as seen from Earth.

This is what the shadow of Phobos looks like on the ground. The shadow is distorted because of the angle of the shadow cone projected onto the surface. As you can tell from the shadowed crater rims, sunlight is coming from around the 11 o’clock position:

The shadow moves across the surface with a speed of roughly 7200 kph from west to east, so Phobos transits never last very long — only 30 seconds on average. On Earth total eclipses can last up to 7½ minutes, though they average around a few minutes.

But the brevity of these events is offset by their frequency. Once every orbit there will be a Phobos transit visible from somewhere on the surface. And since its orbital period is a mere 7 hr 39 m, if you’re lucky, you could see two transits in the same day!

If you want to see the geometry of the orbit, and why these eclipses are seen so often from the equator of Mars (Perseverance is positioned near latitude 18.5° N), see my July 18, 2022 post.

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