Moving into line

Over the next several evenings you can see Venus sliding past the Twin Stars, Castor and Pollux.

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Close-up:

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You could watch for three markers in this progress: the Venus-Castor “appulse” or closest approach, the Venus-Pollux appulse, and the Venus-Pollux-Castor straight line.

Venus will pass more than 7 degrees south of Castor in the night between May 26 and 27. The closest moment is just about exactly between these two dates: 0 hour Universal Time (midnight in Europe, 8 PM by Eastern American clocks). But exactness scarcely matters with a gap this wide. The appearance will be little different on the evenings before and after.

The “conjunction in right ascension,” when Venus is due south of Castor, comes more than a day later, on May 28 about 6 UT.

Next, the passage past Pollux. The gap now is only 4 degrees, and the closest moment is May 29 at 22 UT, so this appulse will essentially be taking place on the evening of May 29 whether you are looking from Europe or America. Venus is due south of Pollux less than a day later.

And when does the planet move into the same slanting northwest-southeast line as the two stars? It appears that the evening closest to the moment when this comes about will be that of June 2. The clock time of it will vary depending on your location, and may be something that is easier to estimate than moments of closest approach. Can you hold a straightedge to the sky?

The picture shows the situation on May 29 an hour after sunset, with Venus 4 degrees from Pollux and about 25 degrees above the horizon. The arrow through Venus shows its position on the surrounding dates.

The slightly orange star Pollux is around 35 light-years away. The silvery and slightly fainter Castor (really a close group of six stars) is around 50 light-years away. Venus, which at this time outshines them by five and a half magnitudes, is several hundred thousand times nearer than them, and is on its way nearer to us.

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