Watch the crescent moon at dusk this week, and you’ll see it mount to Venus as if climbing an invisible stairway. Up, up, up she goes, moving a little more than one outstretched fist a night as it revolves about the Earth. On Saturday the two pair up in a lovely conjunction that will put a smile on your face. Phases are caused by the changing angle the moon makes in relation to the sun as it circles the Earth. If you watch closely you’ll see the moon’s phase change from a thin crescent to a fat crescent by the weekend.
Now is also the best time to see the semi-darkened part of the moon illuminated by light reflected from Earth’s oceans and clouds called earthshine. Although it’s visible without optical aid a pair of binoculars really improves the view. With only the slightest magnification you can make out a few lunar seas and even a crater or two in the gray-blue gloom.
Venus reached it greatest apparent distance from the sun of 46° on March 24. It’s now slowly slouching back toward the western horizon. Like the moon, the angle it makes with the sun and Earth during its orbit also changes, resulting in phases. Earlier this week the planet looked exactly like a half-moon except but without any features like craters or mountains. Bright clouds cover the entire planet, the reason it appears smooth and dazzlingly white in a telescope.
The line that divides the sunlit half of Venus from the half still in darkness, called the terminator, appeared exactly straight around March 24th. Now it’s slightly concave (curving inward). Over the coming weeks Venus’s phase will change from half to crescent and also increase in apparent size and brightness as it approaches the Earth.
Meanwhile in the morning sky, three bright planets are still jockeying for position at dawn. Don’t miss a chance to see Jupiter, Mars and Saturn this week as they continue to undergo multiple conjunctions with each other. A conjunction occurs when two planets line up closely to the north and south of each other. The trio will span just 6.5° of sky tomorrow, a pretty sight to wake up to.