On the night of October 18, 2016, the moon occulted the star Aldebaran. An "occultation" simply means that one astronomical object passes in front of another. This is similar to an eclipse and the distinctions between the two are subtle. We really only think of an eclipse involving either the sun or the moon. For a solar eclipse, the moon passes in front of, and blocks the sun, either in part or in full. In a lunar eclipse, the Earth passes between the sun and the moon and the moon enters Earth's dark shadow. The moon seems to disappear because it is deprived of the sun's light. But other objects in the solar system can block and be blocked, and these arrangements are generally called "occultations" if the body that blocks is larger than the body that gets blocked. Usually our moon is the celestial object that does the blocking. The moon is relatively wide as seen from the Earth and can cover a decent patch of the sky (the moon is about half a degree wide). The planets and even asteroids can also cause occultations, but planets and asteroids have small angular sizes, and so the chances of an occultation happening are very small.

Since the moon is relatively wide, and there are stars throughout the sky, technically, the moon is always occulting something. What becomes interesting is when the moon occults a bright object like a star or planet. Bright stars are distributed essentially randomly on the sky, but planets are not. The planets cluster on a part of the sky called the Ecliptic, which is simply the line of the equator of the Earth projected onto the sky. The Earth and all the other major planets are inclined by only a few degrees to the sun and so the ecliptic tends to define the plane of the solar system. All the planets are found within a few degrees above and below the Ecliptic. It just so happens that the moon orbits the Earth in a plane very close to the Ecliptic as well. The moon's orbit is inclined by only 5 degrees. So the moon and all the planets are all travelling through the same strip of the sky a few degrees above and below the Ecliptic, like cars travelling on a freeway, making the chances of them coming close on the sky pretty high. Incidentally, for this same reason, "conjunctions", that is the moon and planets coming close together in the sky, are fairly common as well, happening a few times per year. An occultation is sort of an extreme version of a conjunction, where, rather than merely coming close, one object passes in front of another. On average there are a few occultations of a planet by the moon each year.

Unlike planets, bright stars, are not preferentially clustered along the Ecliptic. Really bright stars are actually rare and ones close enough to the Ecliptic to be occulted by the moon are rarer still. In fact there are only 4 stars as bright as first magnitude that are close enough to the ecliptic to be occulted by the moon. These are Regulus, Spica, Antares, and Aldebaran. Of course, if we include fainter stars, the moon is always occulting something. But the ability to see, with your naked eye, the moon pass in front of and block the light from a star, is something only possible with bright stars. A lunar occultation of a bright star is something that happens only a few times per year. And then just because it happens doesn't mean you can see it from your location. Lunar occultations last an hour or so and happen at essentially random times. So there is no guarantee that an occultation is visible from your location or occurs at night. That's right, the moon is in your sky half of the time, and that includes during daylight hours. Occultations can occur during the day, and while you can see the moon and bright stars during the day if you use a telescope, it is a less compelling experience than observing them at night. So, the lunar occultation of Aldebaran this October was a rare event that was rarer still when you consider it happened at night and at a time when the moon was visible in my sky. Oh, yes, and then you also have to be lucky enough to have clear weather.

We don't think about the moon moving through space. When we look at it it seems fixed. Of course the phases of the moon are caused by the moon moving in its orbit and changing the angle it makes with respect to the sun. But this appears to happen from day to day and not in real time. So we take the moon's orbital motion for granted. In fact the moon is moving 2,300 miles per hour on average, which is over 4 times faster than a jetliner. It just so happens that the moon is physically about 2159 miles in diameter, which is close to how far its orbital speed takes it in an hour (2300 miles per hour versus 2159 mile across). The moon is about half a degree wide as seen from the Earth, which means the moon moves approximately it's own angular diameter every hour. When you look out at night and see how big and bright the moon is, realize it's a body in motion which moves as far across the sky in an hour as it is wide. During an occultation, this fact becomes more apparent. After all, it's the moon that moves, while the star it occults remains fixed. So occultations last about an hour (the time it takes for the moon to move its own diameter). Actually it can be a bit less than an hour if the path of the occultation doesn't go through the moon's diameter. The first video on the right is a timelapse of the occultation and spans about an hour. You can see the moon move across the star Aldebaran and then it emerging from the other side of the moon. The timelapse spans about an hour. Occultations are great in that they don't take too long and are a great lesson in orbital motion. The 2nd video shows the beginning of the occultation, with the moon slowly approaching Aldebaran and blocking it out abruptly. The 3rd video is of the end of the occultation with Aldebaran appearing from the limb of the moon. It seems to pop out of empty space because that limb of the moon was in darkness due to the moon's phase. The end of the occultation is harder to photograph because, unlike the beginning, you can't follow the star's progress. It just suddenly pops out from behind the moon in an instant and you just hope your telescope is pointing at the right spot. All in all occultations are fun and easy to observe with minimal equipment. Just your eyes or a pair of binoculars is all you need, but the view is enhanced with a telescope and if you use a video or still camera. I used a StarSplitter Video Astronomy Solution which allowed me to observe the occultation through an eyepiece while at the same time I shot the video that appears in this blog entry.

Timelapse of the October 18, 2016 lunar occultation of the star Aldebaran.  The timelapse spans about an hour and was taken with a Canon T2i digital camera mounted on a 4-inch refractor telescope. The moon is overexposed to show the star Aldebaran.  You can get a sense for how fast the moon moves in its orbit. The star Aldebaran is fixed and the moon moves in front of and then away from Aldebaran.

The beginning of the lunar occultation of Aldebaran from October 18, 2016. Aldebaran is the star near the moon's limb. The moon's orbit carries it closer and closer to occulting Aldebaran until it blocks it completely and suddenly. Taken with a StarSplitter Video Astronomy Solution on an 8-inch telescope

The end of the lunar occultation of Aldebaran. After being behind the moon for 37 minutes, the moon moves in its orbit enough for Aldebaran to be visible again. It pops out from behind the moon suddenly.