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Lunar Occultations South Shore Astronomical Society Since 1958 Lunar-See Challenge Series – Issue #1 Now You See Me, Now You Don’t Lunar Occultations ‘As the world turns’ isn’t just the name of a long-running but now defunct daytime soap opera, it’s also a phrase that describes an astronomer’s life to a ‘T’. Every day we watch as everything in the sky marches from east to west, a direct result of the Earth rotating on its axis. But what is less evident is the proper motion of many of the objects that we observe, and in this case we’re talking specifically about the Moon. Every day that it’s visible in the sky, we watch the Moon rise in the east and set in the west, just like pretty much everything else up there. That’s what’s called apparent motion. But what you may not be aware of is that the Moon’s proper motion is actually west-east, as the Moon progresses in its orbit around Earth. It’s this proper motion and orbit around Earth that creates the phases that we see and the changing of the tides from day to day. It takes the Moon 29 days to complete a set of phases in what is known as the synodic orbital period. This composite of three images taken from the same location, looking west and almost exactly 24hrs. apart clearly shows the proper motion of the moon from west to east over the course of 48hrs. The Moon moves W-E at about 12° per day, which equates to about .5° per hour. It’s this proper motion of the Moon that makes lunar occultations possible, and they’re one of the most exciting things we can see in the eyepiece. To begin this conversation about occultations, I’ll pose the following rhetorical question; when does an otherwise ordinary star become something very, very special in the eyepiece? As amateur astronomers we spend a lot of time looking at stars, with many – if not most of them – remaining entirely anonymous to us from start to finish. Yes, some are brighter than others, and yes, some exhibit more color than others, but for the vast majority of our observing time they don’t leave us exclaiming things like ‘oh, wow’, or ‘that was amazing’! Stars are amazing of course, but typically speaking they have to do something really special to elicit a strong emotion from the observer. Of course there are times when they do leave us making exalted exclamations, and that’s part of the allure of astronomy. There are beautiful double stars and gorgeous star clusters. There are stars gone nova and stars gone supernova. There are stars being born and stars in their death throes. There are throbbing stars that change in brightness so dramatically as to be noticeable to the naked eye alone one moment, and not the next. And there are stars that actually appear to move around on the firmament of the starfield. But the gist of this prose is to discuss when a star that does none of those things gets its fifteen minutes of fame, when it stands head and shoulders above all those around it, when it is literally the star of the moment. And that, my friends, happens when a star is occulted by the Moon. Lunar occultations are interesting events. An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The most common form of occultation in the astronomy realm is when the Moon passes in front of a distant star as it makes its way around the Earth every lunar month. As the Earth makes its way around the Sun every 365 days, different stars appear to get into the Moon’s path, sometimes bright ones. Lunar occultations of bright stars are celebrated events, typically attracting widespread attention and observing (and imaging) of the occurrence by amateurs and professionals alike. Other objects such as planets, asteroids and dwarf planets can occult stars as well, but these are typically much less frequent and usually more difficult to observe than lunar occultations. The planets also occasionally occult one another, but these are exceedingly rare events. The Moon is our solar system’s occultation specialist. The single best time to see the Moon run over stars and spit them back out again is during a total lunar eclipse. If you’ve ever observed the moon through a telescope during totality then you know what I’m talking about - there are stars filling the field of view in the eyepiece all around the Moon. To a veteran lunar observer seeing this for the first time, it can come as quite a surprise. That’s because even though they may have looked at the moon time and again through their telescopes, typically speaking there weren’t many stars in the field of view to accompany the Moon. The Moon’s brightness has a tendency to wash them out and render them invisible, especially if the transparency is compromised and contrast suffers. And that’s where the impetus for this prose comes in – occasionally a star, or group of stars, are bright enough to be seen in the same field of view with the Moon even when there’s no eclipse. And if they’re in the Moon’s path of travel, you may get to see an occultation event. It’s the unexpected ones that are particularly exciting, not unlike walking down the sidewalk and finding a hundred dollar bill. You didn’t expect it to happen, but then it did and it made you happy. And just like how that hundred dollar bill suddenly makes you richer in a financial sense, a random star that suddenly makes itself known by being a part of an occultation event makes your observing session richer as well. Allow me to cite some examples; In May of 2019 the Moon plowed over the Beehive cluster in an event that was anything but unexpected, but then I’m sometimes prone to moments of obliviousness and so I honestly wasn’t aware that it was happening when it was. Imagine my surprise then when I set a scope up for a quick peek at the Moon through a lightly overcast and hazy sky, and all of sudden there’s M44 looking back at me too! Sadly, I didn’t get to see the occultation of the star cluster in its entirety thanks to those clouds getting thicker, but I did get to see part of it and it was exciting. Using my cell phone I was able to capture the somewhat crude picture of it shown below to memorialize the moment. In this picture of the Moon and the Beehive cluster as seen through the eyepiece of my telescope, the dark portion of the Moon has been brightened by a combination of earthshine and the extended exposure necessary to render the stars on the image, while the illuminated phase has been completely blown out by overexposure. A haze in the sky is also contributing to the overall glare in the scene. Shortly after having this serendipitous moment at the eyepiece, I sat down at the computer to consult a planetarium program to help identify exactly what I’d just seen. The Beehive cluster, aka Messier 44, doesn’t contain any exceptionally bright stars. With the brightest coming in at around magnitude 6 you wouldn’t guess that it’s an easy naked eye object under darkish skies, but there are lots of them so collectively it’s a bright cluster. During the time period that the event was visible to me the Moon had still not reached the meat of the cluster where the brightest stars reside, but there were still plenty of visible stars. 38 Cancri, coming in at about magnitude 6.65, is the brightest star in the field of view of my capture. In this Cartes Du Ciel screenshot that matches the time and field of view of my picture above, we can see the darkened portion of the Moon just encroaching into the outskirts of the Beehive Cluster. It’s always exciting to see the stars disappear suddenly as the darkened limb of the Moon snuffs them out. How quickly this happens cannot be explained in words that would have sufficient impact. You really have to see it for yourself. The Moon is relatively close and moving relatively fast. It happens very quickly! The yellow arrow indicates the direction of the travel of the moon. It’s fun to get a sense of its movement in the eyepiece. The closer the Moon gets to a star the faster it appears to be moving. Moving on to another surprise occultation event, this time the unexpectedness of it was more understandable. There was really no reason for me to be looking when I was, other than that’s what I do. This one is definitely a case of being in the right place at the right time. But first let’s talk about supper. We’re meat eaters here in the McCabe household, and over the years we’ve tried lots of different ways of preparing our meats. We’re not finicky – red meats, poultry, fish – it’s all good. We’ve got lots of delicious recipes and whatnot, and in general we have been quite satisfied with the outcome of our meals over the years. One meat, however, has been a perennial challenge to get perfectly right on a consistent basis, and that has been pork chops.
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