Watch A Unique ‘Two-Step’ Occultation Of Porrima Tonight

The half-moon occults the star Porrima as viewed from Cleveland, Ohio this evening. Time is EDT. Created with Stellarium

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.

At left is the view of occultation from Minneapolis in a twilight sky. Porrima is occulted in Denver at 8:23 p.m. local time just before sunset and reappears on the bright side at 9:21 p.m. Created with Stellarium

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.

This map show the visibility zone for the occultation. Click it for a list of cities and times of the star’s disappearance at the dark limb of the moon and its reappearance at the bright limb. Times are shown in UT (Greenwich), so subtract 4 hours for Eastern, 5 for Central, 6 for Mountain and 7 for Pacific. Credit: Occult 4.2

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.

Through a telescope at around 150x, Porrima reveals its true character as a close double star. As the moon moves east and occults the star, they’ll get covered one after the other about 5 seconds apart. To see the occultation, just click here for the table of cities and times and start looking before the occultation begins. You can stick around to see it reappear on the bright side of the moon about an hour later. Created with Stellarium

Although you can watch the event in binoculars, to appreciate the double-your-pleasure aspect, a telescope is a must. Clear skies!

4 Responses

  1. Richard Keen

    Bob, that was way neat – thanks for the reminder!
    I set up the 12.5-inch in a clear, cool, stable evening. At 100x the double was easy and distinct. This is the first time in 50 years it has looked that nice. Fifty years ago it was a piece of cake in a 3-inch reflector, but then the two stars pulled closer and it was impossible even with the 12-inch. Over the past few years it’s been a blurry elongated blob that sometimes split, but our infamous mountain wind seeing never allowed for anything better. Today’s occultation occurred right at sunset, and the bright sky reduced the glare and the double looked like two brilliant pinpricks against the daylight blue. So that was the first thrill.
    Then, the northern star blinked out, and the survivor went 6 seconds later.
    Now dinner, and back out in 35 minutes for the reappearance.
    I’ll look at some lunar geography while I’m at it.

      1. Richard Keen

        The reappearance was just as clear – and just as neat. Back at 100x, the two stars appeared 5 seconds apart. It looked a bit like a Chesley Bonestell painting of a double star rising above the horizon of a planet, except in motion.
        A few minutes later, another fairly bright star – magnitude 5 or so – went behind the dark limb at about the same place Porrima did an hour earlier.
        Sorry about your clouds.

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