If the nights are growing too chilly for you to go stargazing, then take heart. You can do a little daytime astronomy this week when the waning crescent Moon passes in front of the planet Venus on December 7, 2015. The occultation will occur in daylight skies and can easily be enjoyed with a pair of binoculars or a small telescope. The event is visible in all of North and Central America. In the rest of the world, the Moon will not pass in front of Venus, but it will lie very close to the planet and guide your eye so you can see Venus in broad daylight.
The trick to seeing this event is to find the Moon during daytime. It will appear as a waning crescent, just 13% illuminated, about 45º west of the Sun. In dark-blue sky it will be easy to find even without binoculars. In slightly hazy sky, the Moon will be harder to find. Sweep the sky with binoculars to try to spot it. As an alternative, you can find the Moon just after sunrise before the sky gets bright and follow it periodically for the next few hours before the occultation. Once you find the Moon, before the occultation starts, Venus will be easily visible in binoculars. If you have very clear and blue sky, you may be able to see Venus with your unaided eye in the daytime sky.
The occultation of Venus occurs as the Moon slowly moves west to east as it moves around the Earth. (It also has an east-to-west apparent motion as the Earth turns on its axis). So because the eastward limb of the Moon is lit as it wanes, Venus will disappear behind the bright limb of the Moon as in the image above. Depending on an observer’s location, the planet will disappear for about an hour.
When will the occultation begin? Well it depends where you are. Again, it’s only visible in North and Central America. In Montreal and Toronto, Venus disappears behind the Moon at 12:38 pm and 12:35 pm local time, respectively, and reappears at 1:35 pm and 1:36 pm. In Chicago, the occultation runs from 11:18 am to 12:32 pm (local time). In Vancouver and Los Angeles, it goes from 7:53 am and 8:04 am, respectively, to 9:23 am and 9:53 am. Earlier in the morning is better because the sky tends to be darker and bluer. Observers in Alaska and Yukon can see the occultation before the Sun rises.
But even if you miss this lunar occultation, or if it’s not visible in your locale, find the crescent Moon on the morning of December 7 and, with binoculars, look for Venus very close by. Then lower your optics and see Venus in the daylight for yourself. It’s often visible in the daytime once you know where to look.
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