Enjoying EarthSky? Subscribe.

263,087 subscribers and counting ...

Occultations galore on September 18

Above photo: Lunar occultation of Venus by Ravindra Aradhya on February 26, 2014

Tomorrow – September 18, 2017 – the moon will occult (cover over) three planets (Venus, Mars and Mercury) and one first-magnitude star (Regulus) in less than 24-hours time. This will be the first time since March 5, 2008, that three planets have been occulted by the moon in less than one day. The next time won’t be until July 24, 2036. Overall, this string of planetary occultations may be more of an academic than observational interest, because much of the world is not particularly well-situated for watching even one of these four occultations.

But don’t let that stop you from waking up early – say 90 to 60 minutes before sunrise – to get an eyeful of the moon, morning planets and Regulus all lining up in your eastern sky. We give you fair warning. Only the moon and Venus will be easy to spot in the predawn/dawn sky, though it shouldn’t be all that difficult to spot Regulus a short hop below Venus before dawn. It’ll take a more deliberate effort to catch Mercury and Mars near the horizon as the predawn darkness is giving way to morning twilight.

You’ll almost certainly need binoculars to see Mars – and possibly Mercury.

Click here for recommended almanacs; they can give you the rising times of the sun, moon, planets ainto your sky.

This chart is specifically for North American mid-northern latitudes. From the world’s Eastern Hemisphere, look for the moon to be offset toward Venus and Regulus before sunrise September 18. The occultations of Venus and Regulus will have already passed by the time the moon and planets enter into the North American morning sky on September 18, 2017.

On September 18, the moon will occult Venus first (1 UTC), the star Regulus next (5 UTC), then Mars (20 UTC) and finally Mercury (23 UTC). To see any one of these occultations, however, you have to be at just the right spot on Earth. Even at that, the occultations of Regulus, Mars and Mercury will be extremely hard to observe because they occur in a daytime sky. Undoubtedly, you’ll need a telescope to see any of one these lunar occultations – that is, if any one of them even takes place in your sky.

A partial glance of the 2017 occultations via Guy Ottewell’s Astronomical Calendar 2017. Note that all four occultations on September 18, 2017 take place in a daytime sky.

Although the occultation of Venus happens during the daylight hours, as well, sharp-eyed observers can actually see the moon and Venus at daytime with no optical aid. We’re expecting some of our friends in the Southern Hemisphere – Indonesia, Australia and New Zealand – to see this occultation in their daytime sky on September 18. Remember, if you can see the moon – but not Venus – in the daytime sky, aim binoculars at the moon to spot nearby Venus in the same binocular field. Click here to find out the occultation times for numerous localities in Indonesia, Australia and New Zealand in Universal Time. Here’s how to convert Universal Time to local time.

Worldwide map via IOTA. Everyplace in between the dotted red lines sees the lunar occultatation of Venus in a daytime sky. Click here for more information.

Use the waning crescent moon and the dazzling planet Venus, the brightest and second-brightest heavenly bodies of nighttime, respectively, to seek out the grand alignment of the moon, planets and Regulus in your eastern sky some 90 to 60 minutes before sunrise. Good luck!

Bruce McClure

MORE ARTICLES