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. In the oven or on the stovetop, pork chops all too often find a way to get dried out. It’s as though that’s the nature of the cut. Until recently, that is. I don’t know if I’d be considered a Grillmeister on a global scale, but I’m certainly a grill lover on my patio. And we’ve finally figured out the pork chops. The wife handles the pre-prep and marinades, and I handle the cooking, and they’ve been consistently coming out very yummy -finally!
So that’s what I was doing on the evening of the 22nd of October 2020 when I experienced another serendipitous observing event. I had just lit the grill and was letting it warm up, when I decided to pull a scope out and take a quick peek at the Moon and planets while I waited. There wasn’t anything special going on except that Luna, Jupiter, and Saturn were arranged in a small triangle on the sky. The Great Red Spot wasn’t visible at the time, the Galilean Moons weren’t doing anything special and the Moon was at 43% waxing crescent. I was just looking for looking’s sake.
And then it happened. I was simply carousing around and looking at the Moon when a star suddenly and quite vividly appeared on the northeastern limb of the orb of green cheese. By all measures it was just an ordinary star that otherwise wouldn’t attract much attention, but in this moment it was stunningly beautiful! I watched for a few minutes as the Moon slowly pulled away from the star, the gap steadily growing as the respective motions of the objects played out in real time. It’s my opinion that a moment like that is of the most beautiful things an observer can witness in the eyepiece, and it has the power to make your whole day better.
As I usually do, a little later on when I had the time I did a little research to see what star was creating such a scene in my field of view. As expected, it wasn’t anything that would garner much attention in any other setting. It didn’t have a fancy name like Zubenelgenubi, it didn’t boast the brightness of a Summer Triangle star, and it didn’t have a lurid history of threatening to blow its guts out all over creation like Betelgeuse promises to do someday. It was just little old 52 Sagittarii.
I use the term ‘little’ in a figurative sense. 52 Sgr is a magnitude 4.6 star about two times the size of and three times the mass of the Sun, with a luminosity about sixty times that of the Sun. It’s located about 190 light years distant in the constellation Sagittarius, and is moving toward us at a velocity of about 19km/sec. 52 Sgr also has a partner, a 9.2 magnitude companion and the system is separated by about 2.4” from our perspective.
Before the accidental occultation observation I knew nothing about 52 Sgr. Now I know that it’s going on one of our observing objectives lists next July or August as a double star target. With a nearly five magnitude difference in brightness, a close 2.4” separation and a culmination altitude of just 23* above the southern horizon, it should prove to be a nice challenge to resolve. It’s funny sometimes how these things come to you. One minute you’re completely ignorant about something, and the next you’re like “hey, look what I know”.
In this Stellarium screenshot from the time of my observation you can see the emergence of 52 Sgr from behind the northeastern limb of the Moon. Occultations have two main elements – the disappearance moment and the reappearance moment. A grazing occultation may have multiple disappearance and reappearance moments due to the mountainous and uneven profile of the Moon’s limb.
In the same way that witnessing the disappearance or reappearance moment cannot be effectively described to a person that’s never seen it, the image above can in no way do justice to the experience of seeing an occultation event with your own eyes. There are basically two ways to do that; look up predicted occultation events and make the effort to see them, or just get out often enough that you get lucky and happen upon one in progress. I’ve done both.
You’ve just heard about a couple of my serendipitous experiences, and those are just about getting lucky and being in the right place at the right time. But then there are the events that we plan for, something we learn about that will be happening at a specific time and we make it a point to catch it. Here’s one from my archives; on September 10th, 2014 the mighty gas giant Uranus was slated to be occulted by the moon as seen from our area. Actually, only the reappearance would be seen, as the disappearance happened when the duo was below the horizon. And even the reappearance wouldn’t be an easy catch. First of all the timing had it happening very low to the eastern horizon, and secondly Uranus is a full 18+ magnitudes dimmer than the moon! A little distance between the two would be necessary to see them both of them at once. I traveled to a pre-planned observing site with a low eastern horizon and trained my scope on the moon. Ultimately I was successful in witnessing the reappearance event, but it was definitely a challenging one.
In this image from the September 10, 2014 Lunar occultation of Uranus, the pale blue/grey gas giant can barely be seen well distanced to the right of the bright lunar limb. I could actually see the pair in the eyepiece when they were much closer together, but it took me a while to figure out what camera settings were needed to finally expose Uranus in the image. Suffice it to say that it required significantly overexposing the moon to get the planet to show in the picture.
In another planned observation, on April 10th, 2016 Louis Gentile and I met at Centennial Field in Norwell to observe a complete lunar occultation of Aldebaran. The .85 magnitude alpha star of the constellation Taurus would disappear behind the dark limb of the waxing crescent moon in broad daylight, then reappear from the bright limb about an hour later in a dark sky. We got lucky with clear skies and perfect positioning of the objects in the sky, and we enjoyed a spectacular observation.
You typically don’t need a large telescope to observe occultation events, but it certainly doesn’t hurt. Shown in the picture above are my 5” refractor, and Louis at the eyepiece of his 18” Newtonian reflector.
I wanted to be at the eyepiece for the disappearance of Aldebaran behind the dark limb of the moon, and for the reappearance too for that matter, but I put a camera on the scope shortly after the reappearance and captured the duo as the Moon slowly pulled away from Alpha Tauri, the bright star otherwise known as Aldebaran.
Occultations are not necessarily rare events, but they do have a tendency to be viewable from only small swaths of area on our fair planet. That’s because if you move a significant amount in any direction from any given place, the perspective that you’re looking at an object from changes significantly as well. This is why it’s nice to have someone working out the technical details about event viewability beforehand. That’s where The International Occultation Timing Association comes in. It’s at this website that you can learn everything you ever wanted to know about lunar occultations and more, including predictions for many major city locations in the United States. The website is very comprehensive, if not a bit dated and dauntingly laden with reams of text and complex tables. For data buffs this is a dream site, but for someone like me who prefers a screaming headline that says ‘Hey, guess what? There’s a bright occultation occurring in YOUR area tomorrow and here’s all the info you need to see it’, the format can seem bit clinical. Alas, I’m probably not cut out for a profession in astrophysics after all, but darn I can cook a mean pork chop!
Click here to plan your next occultation viewing: http://www.lunaroccultations.com/iota/iotandx.htm
Happy Viewing,
Mike McCabe