Moon over Aldebaran

¡SkyCaramba! Weekly astronomy blog for the week ending February 21, 2015

Happening this month is an astronomical event that happens just about every month for almost four years. Then it takes about 15 years off. And then it happens again about every month for almost four years. On February 25 will be the second of 49 occulations of Aldebaran by the moon.

The moon passes directly in front of the orange star in the Hyades every 27.3 days from January 2015 to September 2018. None of the events will be visible from all of the earth. Indeed, some won’t be visible at all because they happen during the daytime. When they are visible, they’ll be visible in a broad band somewhere on the planet.

This business of the moon occulting the star so many times in a row and then missing it consistently for 15 years has to do with a long lunar cycle. The moon’s orbit is at a 5° angle to the ecliptic (the earth’s orbital path and also the path the sun takes through the sky). The lunar orbit path slowly shifts around the sky dome during an 18.6 year period. Even though the moon travels a slightly different path each month, it’s wide enough and the orbit path moves slowly enough to repeat the occultation many times. And of course, the earth is wide enough for the occultation to be seen somewhere on it too.

Aldebaran isn’t the only star the moon regularly occults and then regularly misses. Many other stars it passes near are occulted sometimes.

During the 18.6 year lunar cycle, the moon’s inclination to the earth’s equator varies from a minimum of 18.5° as it will be for most of 2015 to a maximum of 28° as it will be in 2025. Although the moon isn’t actually standing still in reality or apparent motion, these extremes have come to be called the minor lunar standstill and the major lunar standstill, respectively.

If you want to find out if any of the 49 occultations in the current Aldebaran series are visible where you are, consult the image on this page. Click on it to see the full size version. If your location is between red dotted lines, it will be a daytime occultation. Nighttime occulations are those between white solid lines. You can also find more information about them at the link below.


Visibility maps for the moon occulting Aldebaran 2015 to 2018

Image made with Win Occult.
Click here for the full size image.