Bull’s Eye

The Moon is aiming for Aldebaran, the bright reddish “eye” of Taurus the Bull.

Moon occults Aldebaran

On Friday morning, Oct. 2, the Moon will occult (hide) the star, as seen from western North America before daybreak.

The event is at about 13 hours Universal Time, which is 5 AM local time in the western states, called 6 AM PDT by clocks on the ever-confusing Daylight-Shifting system that is imposed on us.

Here (detail from a page in Astronomical Calendar 2015) is how the track of the occultation – the “shadow” of the Moon cast by Aldebaran’s light – sweeps across the Earth.

Moon occults Aldebaran
On the left: Earth as seen from Moon, with the track of the “shadow” of Aldebaran. On the right: Moon as seen from Earth’s center, with the track of the star past it.

This occultation track crosses the Pacific during the night, western Canada and the western US in the hour before dawn, and the rest of North America during day, when the star can’t be seen (except by some extreme methods).

So our sky scene is for longitude 120 west and the latitude of the US-Canada border. For Alaska, mentally draw the horizon 12 degrees or so higher (farther north); for San Francisco or Los Angeles, 11 or 15 degrees lower.

The Sun will rise about an hour later – around 6 local time (7 by clocks), a few minutes earlier if you are farther south.

Here is a closer picture, cribbed from the website of Curt Renz, of the event as seen from San Francisco:

Aldebaran occulted by Moon, Diagram by Curt Renz.

On the left, Aldebaran is about to meet the bright advancing limb (edge) of the Moon, at 6:17:42 PDT. In the middle image, the hidden star is the nearest it gets to the center of the Moon. On the right, it pops out at the dark edge, at 7:16:48 PDT. And this is the more exciting but also harder moment to watch for, since the star is hidden until it isn’t, and the sky has become brighter by an hour.

Seeing a bright star born from a point in the sky that the Moon has just passed over is exciting. But watching and timing occultations – not only by the Moon, and not only of bright stars – is a serious scientific pursuit for a fraternity of enthusiasts, and to find out about this you could look at www.occultations.org.

1 thought on “Bull’s Eye”

  1. We had clear weather in San Francisco yesterday, so I was able to observe this occultation, through mounted 11×56 binoculars. I lost Aldebaran in the Moon’s glare about 4 minutes before the predicted time of disappearance. But the reappearance of Aldebaran was quite dramatic. One moment, no star. The next moment there was Aldebaran shining weakly but definitely in the daylit sky.

    And I still made it to work on time.

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