Hyades occultMN_FEA

Half-moon’s Heart Set On The Hyades

The half moon, called first quarter because it's moved a quarter of the way around its orbit by tonight, shines from high up in Taurus the Bull this evening. Stellarium
The half moon, called first quarter because it’s moved a quarter of the way around its orbit by tonight, shines from high up in Taurus the Bull this evening. The vertical edge of the moon, called the terminator, advances from right to left from new moon to full. Stellarium

Tonight, the half moon enters the constellation Taurus about a fist from both the Hyades and Pleiades star clusters. After crescent phase, first quarter is my favorite because I like seeing a perfectly straight-up-and-down terminator. That’s the line that divides the sunlit half of the moon from the half still in darkness. The terminator neatly bisects the moon twice during the its revolution around the Earth — at first quarter phase 7 days past new moon and again at last quarter phase 7 days before the new moon.

The moment of half-ness occurs around 1:45 a.m. CST Monday morning, when the moon will have set from my location. Since I’ll be watching closer to 8 and 9 p.m. the terminator should appear ever so concave or slightly less than half. I wonder if I’ll be able to see the difference. It’s all moot anyway at least for me, since clouds are expected to hang tight this evening. What will you see?

This map shows the path of the moon through the Hyades cluster on Monday night (Feb. 15) as seen from the Minneapolis, Minn. area. Times shown are several minutes before the stars Gamma Tauri and the pair of Theta-1 and Theta-2 Tauri are covered by the moon. Stellarium
This map shows the path of the moon through the Hyades cluster on Monday night (Feb. 15) as seen from the Minneapolis, Minn. area. Times shown are local and several minutes before the stars Gamma Tauri and the pair of Theta-1 and Theta-2 Tauri are covered by the moon. To see the occultations all you need is a small telescope. The Hyades is the closest star cluster to Earth at a distance of 153 light years. Stellarium

Tomorrow night, the terminator will have moved further to the left (or east in lunar parlance) on the moon’s shiny disk and appear obviously convex or slightly bulged out. Crescent phase lasts 6 days as does the time between quarter and full called gibbous. New moon, the quarter phases and full linger but a single day like the proverbial mayfly.

The moon occults Aldebaran at 1:05 a.m. PST on Tuesday morning Feb. 16 very low in the northwestern sky. Stellarium
The moon occults Aldebaran at 1:05 a.m. PST on Tuesday morning Feb. 16 very low in the northwestern sky. Stellarium

Also on Monday night (Feb. 15), the moon will pass directly across the Hyades star cluster. From many places across the U.S. and Canada, it will occult or temporarily cover up some of its brightest members. Folks living from central to southern California and in Hawaii will even get to see Taurus the bull’s twinkly red eye, Aldebaran, disappear around 1 a.m. and 11 p.m. respectively.

Aldebaran’s a bright, first magnitude star, so the occultation might be visible with the naked eye as the moon’s dark edge approaches and then blanks it from view. If the glare of the moon makes Aldebaran difficult to spot, binoculars will show it with ease. For all the rest of us living further east, we’ll get to see other Hyades stars covered, but Aldebaran will have set by the time the moon occults it.


See how fast it happens in this video of the January 20, 2016 Aldebaran occultation. Star disappears 32 seconds in.

Occultations happen very quickly. The star seems to hover along the moon’s edge (called the “limb”) for a minute or two and then disappears in a barely a blink of an eye as if swallowed by a black hole. About an hour later, the star reappears along the moon’s bright limb.

Here are links to check on the disappearance and reappearance times of several of the Hyades stars from a variety of different cities. Remember to subtract 5 hours from the times shown (UT) for Eastern Standard, 6 hours for Central, 7 for Mountain and 8 for Pacific: