Thursday, December 10, 2015

20151002 OR Lunar Occultation of Aldebaran

I noticed in S&T’s October issue (page 49) the moon was to occult of Aldebaran the morning of October 2, 2015. I checked the times and found they would fit nicely in my morning schedule. Disappearance was at 6:17am, reappearance at 7:17am. That would leave enough time to get up, observe the disappearance, have breakfast and get ready for work, observe the reappearance and leave for work right after. Luckily the sky was clear!

I left my Astroscan in the kitchen overnight so I just needed to bring it outside to observe; and bring it in afterward. I went outside shortly after 6:00am (still in my bathrobe!) and quickly found Aldebaran fairly wide from the bright side of the moon (@ 89x). It was nice to see Orion, the wintertime favorite, high in the sky with Canis Major prancing at his heels. I looked at the Orion Nebula; it was there, and I could see the Trapezium, but transparency was not very good with some thin fog in the air.

Back to the moon; Aldebaran was now noticeably closer. I watched as the moon moved perceptibly eastward toward the star – which was interesting since in the eyepiece the whole scene drifted the opposite direction, to the west. It was heading toward Aldebaran at a slight angle, from the telescopic SW to the NE. The effect would have been more dramatic if I had a tracking mount and centered the view on Aldebaran. The moving moon recalled my experience last week seeing shadow and light changes along the terminator; it’s wondrous to see change happen in the sky.

Now the moon’s limb was nearly touching the star; it seemed to float inside the narrow, slightly out of focus band of light my telescope presented of the limb. Then the star winked out. It was so sudden! And there was no bringing it back, no matter how I moved the scope around, looking for the lost star.

I came back outside at 7:10 to find my eyepiece had fogged over – I forgot, it’s now the time of year when we have to manage our equipment for dew! I used a different eyepiece with a wider view (38x) and could see the whole moon. The sky was robin’s egg blue, no stars were visible. I stared at the moon’s nocturnal area and waited, waited. I checked my watch—I was nervous I had missed it—then peered back in the eyepiece: Aldebaran was back in view. In the two seconds I looked at my watch, I had missed the instant of reappearance! The lesson is, be patient. Aldebaran was just beyond the edge of the nocturnal part of the moon, and had followed that south-westerly angle I noticed just before it disappeared.

This was my first lunar occultation of a star, and it was fun. I now have another kind of observing project to add to my backyard observing routine: double stars, planets, asteroids, the antics of the Galilean moons, shadows along the lunar terminator, and now lunar occultation. It would be fun to see a star’s disappearance and reappearance under high power, especially if it happens near mountains on the moon’s limb: I imagine it would be like watching a sun set and rise on earth.

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