Stargazers across most of mainland USA will get a rare chance to see one of the brightest stars in the night sky disappear behind the Moon tonight when there is an occultation of Aldebaran.
Aldebaran is the most brilliant star in the constellation of Taurus, the Bull. It is one of just a handful of magnitude 1 stars in the heavens.
The Moon will be approaching what is called First Quarter, when it has a “half-moon” phase. This means that Aldebaran will disappear behind the dark limb of the Moon, making the event more obvious.
And because we see the star as a point of light, due to its great distance, plus the fact the Moon has no atmosphere, Aldebaran will not fade but will simply blink out in an instant. It will reappear from behind the illuminated limb of the Moon later in the night.
Though the spectacle should be visible with the unaided eye, it will appear more dramatic if you view it with binoculars or a telescope.
Astronomers call this example of celestial hide and seek an occultation. It will occur during the evening hours, but the exact time of disappearance and reappearance will depend on just where you are on the Earth. The Moon is close enough, at a distance of around a quarter of a million miles, that its position against the starry background shifts slightly as seen from places just a few tens of miles apart.
An interesting aspect of this occultation is that a zone which by chance roughly follows the US/Canadian border beyond which Aldebaran will just miss being covered up by the Moon at all. North of this line, no occultation of the star can be seen. This means that no occultation of Aldebaran will be seen from Canada apart from a small region to the south of Toronto.
If you live somewhere along this line, you may witness what is known as a “graze occultation”. If you’re lucky, you may see the star briefly vanish behind the mountains on the northern edge of the Moon, but you could also see no occultation at all. It is hard to predict precisely for a given spot, which is why occultation experts are keen to receive reports from observers on the graze line as to what they saw or didn’t see.
From Los Angeles, Aldebaran is predicted to disappear behind the Moon’s limb at 7.08pm local time (PST) and reappear at 8.27pm. From New York city, it disappears at 11.10pm local time (EST) and reappears at 11.31pm.
You can find full details of how to observe the occultation of Aldebaran, and what time to look in your own area, by visiting this page at lunar-occultations.com. Note that the times given for the time of Aldebaran’s disappearance are given in Universal Time (UT) and you will have to adjust for local time. For example, PST is eight hours behind UT, and EST is five hours behind UT.
If you live near the graze line of the occultation, you can read more about what to expect from your locality by visiting a special page set up by the International Occultation Timing Association.
Aldebaran only rarely gets covered by the Moon because our natural satellite does not follow the same path against the stars on every orbit. This is due to the fact that the plane of the Moon’s orbit is slightly tilted to that of our own orbit around the Sun.
Observers with telescopes may see a number of other stars covered by the Moon on this special evening. That is because the Moon will also be passing in front of one of the closest clusters of stars to us, known as the Hyades. Together with Aldebaran, these stars look like a V-shape in the sky, though Aldebaran is not physically part of the cluster and just happens to lie in the same general direction.
If you are in the UK, Ireland or other parts of western Europe, you will not see an occultation of Aldebaran because the Moon and star will have set before the two bodies come into line. However, observers with a telescope can hope to see some of the fainter stars in the Hyades covered by the Moon during the evening before it sinks too low in the sky and sets.