Occultation of Aldebaran

by Randy Muller

It was clear tonight, so I set my scope outside to begin the lengthy cooldown process. While waiting for it to cool, I read an message from Bill Arnett indicating that the moon was going to occult (ie., block from view) Aldebaran, the lucida of Taurus.

The Moon's Halo

David North reported that the moon was already occulting various Hyades cluster members, so I went outside about 7pm, and there was a dismaying thin layer of high clouds, which caused a lovely 22-degree radius halo around the moon. (I was able to use this 22-degree ring to calibrate my 'fist at arm's length = 10 degrees' measure: I have to include my thumb.)

I knew this wouldn't stop me from viewing the occultation or the moon, but it would stop me from viewing lots of other stuff.

Finally, 10pm rolled around and I set up the Little Feller, my 9" Mak-Cass.

I've seen the moon occult Aldebaran once before, on April 10, 1997. The circumstances were somewhat different: That time, the moon was a waxing thin crescent, and I could clearly see the dark limb as it approached Aldebaran. This time the moon was waxing gibbous, and I could not see the dark limb. In 1997 I used a dob to follow the moon at 142x and it was a little difficult to keep it still.

I was eager to use this scope for an occultation because it has a tracking German equatorial mount.

The Moon

While waiting for the occultation, I examined some features on the moon: The domes west (selenographic) of Copernicus (Rukl 30), near 12N 32W were obvious and very interesting looking, both with the 26. One of them was even casting a shadow.

A little bit south of here were some prominent Rimae: Hippalus (Rukl 53). There were at least three forming gentle concentric arcs.

The Straight Wall (Rupes Recta, Rukl 54) was prominent, and Rima Birt was fairly easy to locate.

Sinus Iridum (Rukl 10) was half in shadow, but the Jura Mountains surrounding it were all ablaze in light, yielding a beautifully eerie effect. Sinus Iridum sometimes looks to me like someone has taken a bite out of the mountains surrounding Mare Imbrium, and the Jura Mountains looked a little like a circular row of small, jagged, pointy shark's teeth.

The Occultation

As the occultation neared, I became worried that I might miss it, because I didn't know exactly what time it would occur, and I could not see the dark limb.

I began viewing with the 26mm eyepiece (119x), but I couldn't get Aldebaran and the moon in the same field of view, so I backed off to the 55mm ep (56x), and framed it nicely.

I was trying to decide whether to track at lunar rate, and watch the star fall onto the moon, or to track at sidereal rate and watch the moon crash into the star. I finally decided on the latter. I put the star in the upper right of the field of view, and the moon was entering the field of view from the lower left.

Gradually the moon drew closer to the star, and I dropped back down to 26mm (119x) and stayed there for the rest of the occultation.

The color contrast between the moon and Aldebaran was very notable and distinct: Aldebaran was a rich golden color. The moon was pale and virtually colorless, showing just shades of gray. I tend to think of the moon as yellowish or tannish or orangeish, particularly when it's low in the sky.

But tonight it was relatively high (a *lot* higher than it has been since I observed it last in August or September), and Selene showed her true colors: None.

My excitement rose as they approached still closer. Periodically I would take my eye from the eyepiece and look at it visually. They were very close. I would get paranoid that I would miss it, so I would return to the eyepiece immediately.

Eventually it happened. I was watching the star approach the terminator, but still a good distance off, when it suddenly went out. It was quite abrupt and a bit shocking. I think I hooted when it happened.

I did not hang around to watch the re-emergence.