We had an almost-occultation when the moon passed very close to the bright star Regulus three nights ago. Tonight, the dark advancing edge of the half-moon will occult Porrima, also known as γ Virginis, for the eastern half of North America and part of the Caribbean during the early evening hours. At the same time, the moon will be only 3° from Jupiter, so this is definitely a twofer.
Porrima is a 3rd magnitude and an amateur favorite because it’s a close double star, each sun revolving about their common center of gravity every 169 years. That’s quick as double stars go, so it’s easy to see the movement of the companion star about the primary in a matter of a few years. Right now, the two are only 2.6″ (2.6 arc seconds)apart. For scale, 60 arc seconds equals 1 arc minute, and 30 arc minutes is the diameter of the moon. So yes, they’re tight but even a 3-inch telescope at 150x can split Porrima into a beautiful pair of equally bright white suns.
When the moon inches up to them tonight, first one of the companions will blink out and moments later the other. That’s the “two-step” part of the event. While some of you may have seen an occultation before, this is unique. There are few such bright, close doubles that moon ever covers.
The occultation happens around 10:30 p.m. Easter Daylight Time when the sky is dark. For the Central Time Zone and some of the southwestern mountain states, it occurs in twilight. Further west, the sky’s either too bright or the sun’s up. By late mid-twilight on the West Coast, Porrima will have popped up on the other side of the moon post-occultation.
Although you can watch the event in binoculars, to appreciate the double-your-pleasure aspect, a telescope is a must. Clear skies!