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Astronomy: It's Just a Phase

12-16-11 apollo08_earthrise
A half-lit Earth as seen by the Apollo 8 astronauts on the moon. Credit: NASA

The moon is not the only solar system object to display phases. Planets can also show a phase, depending on the positions of the planet, the sun, and the Earthly viewer. Even the Earth has phases, as was witnessed by astronauts on the moon.

Venus is one of the easiest planets to observe phases in because it's close to Earth and can be viewed with just a pair of binoculars. Venus and Earth are both spinning along in their orbits, but Venus's forward motion is a bit faster. At the moment Venus is emerging from around the corner of the sun and coming into better view for us on Earth. You can look for the bright point of light in the southwest after sunset to find it. With the sun is positioned between us and Venus, the Goddess of Love exhibits a larger, gibbous phase, as it is now. As Venus catches up with us (being on an inner orbit) and pulls closer to us, its phase will shrink. But even though its phase is shrinking, its overall size, or angular diameter, will grow larger as it gets closer, and the planet will appear brighter.

Venus is currently 86% illuminated, resembling a very distant gibbous moon. By the end of the month, that percentage will have shrunk to 82%, with its lit face continuing to diminish over the coming months. It will hit 50% lit by the end of March but as its lit surface area shrinks, it will grow brighter as it comes closer to Earth in its orbit.

As an example, note below the current date and the stats of Venus compared to how it will be in a few months.

  • Venus on December 16, 2011
    •  magnitude -4.0
    • angular diameter 12.16 arcseconds
    • 86.2% lit
    • 1.37 AU from Earth
  • Venus on March 28, 2012
    • magnitude -4.4
    • angular diameter 23.97 arcseconds
    • 50.2% lit
    • 0.69 AU from Earth

In May, Venus's phase will narrow rapidly as if it were on a starvation diet. The planet is 26% lit on May 1 but less than 1% lit on May 31, as it rejoins the sun in the sky from our vantage point.

-- Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She writes the SkyGuide for AstronomyToday.com. You can follow her on Twitter at twitter.com/Astronomommy.

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