Venus's Daytime Vanishing Act on Monday

Weather permitting, a substantial fraction of the world's population will be positioned to see the dark limb of the thin crescent Moon cover Venus, the brightest planet, on Monday, June 18th. The occultation starts in eastern North America around 13^h Universal Time (9 a.m. EDT in broad daylight). Later in the day it crosses almost all of Europe, again in daylight, then the Middle East around sunset or, spectacularly, in twilight. The last region to see the occultation will be India, in darkness just as the Moon and Venus are about to set (roughly 17^h UT).

In North America only New England and the Canadian Maritimes have a shot, and even here it'll be a tough telescopic challenge. So you'll want to plan your observation well in advance. The Moon will be below the Sun in the brightest part of the daytime sky. And it will be quite thin — just 14% illuminated — and faint. Moreover, the Moon will be quite low in the sky, making it very tough even to locate! Successful observation will require an unobstructed east-northeastern horizon, very clear air, and precise knowledge of the Moon's position.

The farther east you are, the higher the Moon is at the start of the occultation, as shown in the map here. Observers in Newfoundland may be able to watch the entire affair through binoculars, but people in Maine will be lucky to see it through a telescope at moderately high magnification. Observing it anywhere west of Maine will be a major accomplishment.

Venus disappears at 9:05 a.m. EDT in most of New England, 10:07 a.m. ADT in Halifax, Nova Scotia, and 10:41 NDT in St. John's, Newfoundland. It's wise to set up your telescope well in advance to give yourself plenty of time to locate the Moon and Venus. If your scope doesn't have a Go To drive, use the map above to estimate how high above the horizon the Moon will be. Then locate the point on the horizon directly below the Sun, turn 30° left of there, and raise your telescope to the desired altitude. Or you can use a compass to find the correct direction — making sure to correct for magnetic declination. Venus's azimuth at disappearance is 67° east of true north in Boston, 69° in Bangor, Maine, and 71° in St. John, New Brunswick.

Once you've located the Moon, it should be easier to spot Venus just to its lower left.

Keep your eye on the planet as the time for disappearance approaches. The dark side of the Moon is completely invisible during the day, and Venus may seem quite far from the lit crescent when it begins to shrink. Depending on your location, it may take up to a minute or more for the planet to disppear completely.

In most populated areas within the North American occultation path, Venus remains hidden behind the Moon from 30 to 50 minutes. Venus's reappearance on the Moon's bright limb may be easier to watch than the disappearance, with the action happening higher in the sky by then.

Observers in Europe will have a much easier time, as the entire event will take place high in the afternoon sky. The Moon and Venus are the only celestial objects besides the Sun that are routinely visible during the day without optical aid (in very clear air), and it should be a treat to watch one of them disappear behind the other with optical aid.

Sunset will occur while the occultation is in progress for much of the Middle East. A little farther east the occultation happens in twilight; this will be an extraordinary naked-eye event. In the far eastern end of the path in Pakistan and India, Venus's disappearance will happen after dark, very low. That too will be quite a sight — if you have a clear western horizon!

If you capture any images of the occultation, we'd love to see them. Feel free to post your pictures in our online photo gallery. And please e-mail any interesting observing reports. We're particularly interested in observations from North America — both successes and failures — to help us with similar tough-occultation forecasts in the future.

The International Occultation Timing Association has a world map of the occultation and detailed predictions for many cities in Europe and the Middle East. Predictions for Iran and the Indian subcontinent are available from the Inter-University Centre for Astronomy and Astrophysics in Pune, India.

If you live too far west in the United States to see Venus disappear, you may get a chance to see a daytime occultation of Regulus on the afternoon of the next day: Tuesday, June 19th. Although Regulus is much fainter than Venus, this event will take place high in the sky above the Sun, making it easier to observe.