The ‘Great North American Occultation’ sees the Moon blot out Spica Saturday night.
Few events in the sky transpire as quickly as occultations. While the path of the planets may move at a leisurely pace, and the orbits of double stars may be measured in terms of a lifetime or more, occultations are swift vanishing acts.
North American observers have a chance to witness just such an event this coming weekend, when the waxing gibbous Moon passes in front of the bright first magnitude star Spica.
The Moon meets Spica Saturday night. Credit: Stellarium. The Moon is 52% illuminated (just past 1st Quarter) when the event transpires centered around 2:31 (UT) Universal Time (10:31 PM EDT), and most of Canada down through the contiguous United States (CONUS) south into Mexico will witness the entire event; only northwesternmost Canada and Alaska will miss out. The U.S. West Coast sees the occultation occur under dusk skies, while the U.S. Eastern Seaboard and the Canada Maritimes will see the beginning of the event (ingress) underway just before moonset.
The sky on the evening of July 13th. credit: Stellarium. The International Occultation Timing Association (IOTA) has a list of ingress/egress times for select locales inside the occultation footprint here. The Moon moves its own apparent diameter (30′ or half a degree) about once per hour, and waxing occultations are especially dramatic, as the dark edge of the Moon leads the way.
The footprint for Saturday night’s occultation. Credit Occult 4.2. Also known as Alpha Virginis, Spica is the brightest star in the constellation Virgo and is located about 250 light-years distant. A spectroscopic binary with a companion star in a close orbit, Spica is one of the closest stars to our solar system with the potential to explode as a Type II supernova in the next few million years.
Located close to the ecliptic plane, Spica played a role in helping the ancient Greek astronomer Hipparcos to deduce the precession of the equinoxes, as a temple in Thebes built on an alignment with the star in 3200 BC had since changed position with respect to the sky.
Beyond just providing a great show, occultations can reveal unseen companions and even tell us something about the nature of the target object, to include its apparent diameter.
In the current epoch, the Moon can occult three other major first magnitude stars in addition to Spica: Antares, Regulus, and Aldebaran. The Moon could also occult Pollux (Beta Geminorum) up until 117 BC, after which, precession and the star’s own proper motion carried it out of the Moon’s path.
The Moon’s path is a busy one in July. This weekend’s Spica event is part of a current series of occultations of the star by the Moon once per lunation, running out until November 17th, 2025.
Follow that Moon in the next few weeks, we have:
-Wednesday, July 17th: The +84% waxing gibbous Moon occults the bright star Antares (Alpha Scorpii) for South Africa.
-Sunday, July 21st: The Moon reaches Full phase… the July Full Moon is known as the Thunder, Buck or Hay Moon.
-Wednesday, July 24th: The -86% waning gibbous Moon occults the planet Saturn for southeast Asia.
-Thursday, July 25th: The -80% waning gibbous Moon occults the planet Neptune for the western Pacific.
-Monday, July 29th: The -36% waning crescent Moon occults the Pleiades star cluster (Messier 45) for southeast Asia.
All this, from simply watching one celestial body pass in front of another. Keep in mind, these are all part of a busy series of occultation cycles for the Moon in 2024. If skies are clear, don’t miss Saturday night’s occultation of Spica by the Moon.
