Physics Photo of the Week

Physics Photo of the Week

January 24, 2025

The Moon - Mars Occult - Photos by Donald Collins

A week ago Monday (January 13, 2025) the Moon actually passed over the planet Mars in the Sky - visible from about all of North America.  This happened in the evening of full Moon.  In this week's photo we can see the image of reddish Mars about 30 seconds before the Moon completely covered Mars.  This is an occultation of Mars by the Moon.  This is a somewhat rare event that happens once in about 14 years visible from a specific location on Earth.  Events like this are not very spectacular - no major changes in the sky brightness as opposed as happens when the Moon is eclipsed by the Earth's shadow, or the even more spectacular occultation of the Sun by the Moon during a total solar eclipse!  Relatively rare events involving interactions of Solar System bodies with each other or with the stars attract enthusiastic astronomy "nerds".  Solar System bodies' orbits lie within a few degrees of each other.  Because the orbits' are not exactly aligned, the bodies alignments, when they pass each other in the sky during conjuctions, usually miss each other and do not occult each other.  

I captured this event using my 104 mm telescope with my DSLR camera attached - snapping a photo every 5 sec.  I then compiled every 3rd image into a video clip that is played back at about 1/5 second between frames - a speed up factor of 25.  Celestial north is at the top of the photos; celestial east is toward the left.

The video clip above is tracking the Moon's position in the sky.  The individual frames are aligned with the Moon as if the telescope was tracking the Moon and the Moon appears stationary in the video frames.  To our eyes, both Mars and the Moon appear to traverse the sky from east to west as the Earth rotates daily toward the east.  The Moon also orbits the Earth in a 28 day cycle towards the celestial east.  Thus the Moon slowly moves eastward relative to the stars in the sky.  Mars is orbiting the Sun along with the Earth.  The Earth orbits the Sun in a yearly cycle.  Mars, being further from the Sun where the solar gravity is weaker, and the orbit is larger, requires almost two years to orbit the Sun.  In the time that the Moon is occulting Mars, Mars is almost stationary relative the the stars, but still moves very slightly relative to the stars.  Because the video frames track the Moon, the Moon appears stationary in the video and Mars appears to approach and pass behind the Moon. 

Does Mars reappear on the other side of the Moon after the Moon passes Mars' position?  It certainly does.  about 1 1/2 hours after Mars' disappearance, Mars reappears on the opposite of the Moon due to the orbital motion of the Moon relative to the nearly stationary position of Mars.  The egress (reappearance) is shown in the following video clip.

In both videos the last frame is held for about a second to give a pause between the automatic replays.

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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