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Mercury and Venus, the first and second planet from the Sun, respectively, orbit the Sun inside of Earth's orbit. Therefore, these planets act differently in our sky than the planets whose orbits lie outside of Earth's orbit: Mars, Jupiter, Saturn, Uranus and Neptune. Like the Moon, Mercury and Venus exhibit phases, while the planets outside our orbit are always full or close to full.
Mercury and Venus at Greatest Evening Elongations in January
As seen from Earth, Mercury and Venus appear to be tethered to the Sun, and can only swing a maximum distance from the Sun. This maximum angular distance is called a greatest eastern elongation if Mercury or Venus appears in the evening sky. It's called a greatest western elongation if Mercury or Venus appears in the morning sky.
Mercury swings to its greatest evening elongation on January 4, 2009, and Venus swings to its greatest evening elongation on Janury 14, 2009. Because Mercury's orbit is quite eccentric, this planet's greatest elongation can vary from about 18 to 28 degrees from the sun. (A fist held at an arm's length measures about 10 degrees of sky.) However, Venus' orbit around the Sun is almost circular, so its greatest elongation only varies from about 45 to 47 degrees.
Venus' Best Showing as Evening Star in January and February
Venus' best showing in the evening sky in 2009 comes in Janury and February. On January 14, 2009, Venus is 47 degrees east of the Sun. At mid-northern latitudes on this date, Venus sets a whopping 4 hours after the Sun. Surprisingly, Venus still sets a whopping 4 hours after the Sun in late January and early February. As Venus slowly falls toward the Sun, the angle of the ecliptic becomes more inclined, so the two tend to cancel each other out. For the precise setting times of the Sun and Venus in your sky, check out the links on EarthSky's almanac page.
Greatest Elongations Repeat in 8-year Cycles
Look at Venus on the evening of January 14, 2009, and you'll see this world right next to the star Sadachbia (Gamma Aquarii) in the constellation Aquarius. If you view this planet through a telescope, you'll see that this world exhibits a quarter phase (half illuminated), like the first quarter Moon. If you look at Venus 8 years from now - on the evening of Janaury 14, 2017 - you'll see Venus almost exactly in the same spot relative to the stars, right next to Sadachbia. What's more, Venus will once again exhibit a quarter phase. This world will stay out for the same amount of time after sunset and set on the same spot on the horizon. As for the date of Venus' greatest evening elongation, it will come two calendar days earlier - on January 12, 2017.
You can predict Venus's cycles to nearly the day, even without an almanac. For instance, about 36 days after Venus' greatest evening elongation, Venus shines at its greatest brilliance in the evening sky. At this juncture, its waning phase shows Venus' disk about 25% illuminated in sunshine and 75% in its own shadow. Then, about 72 days after greatest evening elongation (36 x 2 = 72), Venus is at its inferior conjunction, passing in between the Earth and Sun, and entering the morning sky. Then, about 108 days after greatest evening elongation (36 x 3 = 108), Venus is at its brilliant best in the morning sky, its crescent again 25% illuminated in sunshine. Then, about 144 days after greatest evening elongation (36 x 4 = 144), Venus is at its greatest elongation in the morning sky, once again at about quarter phase (half illuminated). Unlike the moon, Venus is waxing when in the morning sky, and waning when it's in the evening sky.
220 days after Venus reaches its greatest morning elongation, Venus passes behind the Sun at superior conjunction, to enter into Earth's evening sky. Then, 220 days after Venus reaches superior conjunction, this world returns to its greatest evening elongation. The time period between two successive greatest evening elongations is about 584 days (36+36+36+36+220+220 = 584). Venus returns to its greatest evening (or morning) elongation 5 times every 8 years.
copyright 2009 by Bruce McClure
|Diagram of the Phases of Venus|
December 2008 Feature * February 2009 Feature