Mars Occultation

Unusual Event

The Moon passed in front of Mars on the cold evening of January 13th! In a completely clear sky the full Moon was nicely placed for imaging well above the northeastern horizon. It was a great opportunity to use Seestar to capture the event, and I had everything setup an hour ahead of time. I've used Seestar many times before and always made sure to level it before imaging. This time, for some unknown reason, I forgot the initial leveling. This stupid, careless mistake probably caused troublesome tracking errors later.

My plan was to begin taking individual photos at about 8:20PM EST before ingress and then switch to video as Mars closely approached the lunar limb. At first, photos were taken using Seestar's 1X magnification, but this seemed to make Mars very tiny. For example, here's a cropped 1X image taken at 8:40.

At 1X the entire lunar disc fit within the field of view. As I took a few more photos, I became concerned that Mars' disc would be too small at 1X, so I switched to 2X. Now the whole Moon no longer fit inside the field of view, but Mars' visibility improved. Unfortunately, tracking errors increased making it hard to keep the Moon from drifting away from center. The next image was captured at 9:05, twenty-five minutes after the previous image.

You can clearly see the Mars-Moon separation decreasing. The Moon always moves west to east relative to background stars. Mars reached opposition on January 16th, and, on this January 13th night, Mars was moving in retrograde motion east to west relative to background stars. So the two bodies were approaching each other for a "head on collision" as seen from Earth. At 9:10, five minutes after the previous photo, the separation continued to diminish.

The Moon was moving approximately 35 times faster than Mars relative to the stars this night, so the Moon was like a 105 mile per hour car approaching a strolling pedestrian Mars. By 9:15 the occultation ingress was nearly ready to begin as you can see in the next photo.

At this point, at 9:15, I should have immediately begun recording video. Unfortunately, I waited before hitting the record button at about 9:16. So when the video began, Mars was already just touching the lunar edge. The video below runs for 43 seconds and does show Mars gradually disappearing behind the Moon. The disappearance itself only took about 26 seconds! Unfortunately, it's a bit difficult to follow the disappearance in the center of the bottom lunar limb, but you can see it with some effort. Engage the full screen option. Viewing on a laptop or tablet screen helps. I'm disappointed by the video quality.

More than an hour later, at about 10:27, Mars would emerge from behind the Moon. Since I didn't know exactly where or when Mars would reappear, my second video attempt failed to capture the egress. I began recording about four minutes too late! But I did manage to capture a good photo at 10:31.

Mars' angular diameter was only 14.5 arc seconds in the photo above compared to the Moon's 31.7 arc minute diameter. So the Moon looks 131 times bigger than Mars. Of course, Mars was 5.34 light minutes away while the Moon was only 1.26 light seconds away. Mars was 254 times more distant than the Moon. At the same distance, side by side, Mars' diameter is roughly twice as large as the Moon's.

Finally, although Seestar made it easy to observe this event from indoor comfort, I wonder how much better results would have been if I endured the cold and set up my Nikon Z62 outside on my 130mm refractor. For example, consider images displayed recently in Astronomy Picture of the Day which you can see at the following two links.

https://apod.nasa.gov/apod/image/2501/MoonMars_Sultan_960.jpg 

https://apod.nasa.gov/apod/image/2501/MarsLOc_Jan13.jpg