Caption: A film of a Phobos Transit of the Sun---as seen on Mars, of course.

Phobos (mean radius 11.2667 km) is the larger of the 2 Martian moons. The smaller one is Deimos (mean radius 6.2(2) km).

Phobos is NOT round as you can see in the film.

Phobos' mean orbital radius (AKA semi-major axis) is 9377.2 km, but it is still is a finite, if tiny, disk on the sky as seen from the Martian surface. Note Mars' equatorial radius is 3396.2(1) km.

Here little Phobos makes a valiant effort to make a total eclipse of the Sun, but it fails. It's too small. All it can do is an annular eclipse: i.e., it leaves a annulus or ring of the Sun uncovered.

The transit lasts only ∼ 30 s. The film lasts only ∼ 5 s, and so is time-lapsed.

This transit was observed by Opportunity rover (2004jan25--2018jun10) on 2004 Mar10.

By the by, a transit is when one astro-body passes in front of another one from the point of view of an observer---or crosses meridian---which is like passing in front of an imaginary astro-body.

A transiting body always partially eclipses the transited body, but if the eclipsed part is really tiny, one usually would NOT call the event an eclipse.

Credit/Permission: NASA, 2004 (uploaded to Wikipedia by User:Yaohua2000, 2005) / Public domain.
Image link: Wikipedia: File:PIA05553.gif.
Local file: local link: mars_phobos_transit.html.
File: Mars file: mars_phobos_transit.html.