Look at the sky just after sunset tonight, January 12, and you’re guaranteed to know immediately which point of light is Venus. Hint: It will be the astonishingly bright one. Now get your telescope. As the planet arrives at its greatest angular distance to the east of the Sun, you won’t be able to see surface features, as you sometimes can when looking at Mars, or identify stunning cloud swirls as you might see when viewing Jupiter, because Venus’s thick and unforgiving clouds conceal the planet’s mountains below. But with the help of a telescope, you’ll be able to see that Venus doesn't appear to be a full circle.
Here's why: it's at greatest eastern elongation tonight. What's that? Elongation is the angle between the planet and the Sun from Earth. To understand elongation, point at the Sun as it sets. With your other hand, point at Venus. Simply put, the angle that your arms make is the elongation. Because planets are ever in motion and orbiting at speeds different from one another, that angle is ever in flux. Repeat this process in March and you’ll notice a big difference in the directions of your arms.
The largest this angle will ever get in an orbit is its greatest elongation. When greatest elongation occurs at sunset, it’s said to be at greatest eastern elongation. That's what we have tonight. When it occurs at sunrise, it is at greatest western elongation. For Venus, that will occur this year in June.
Elongation was crucial to Copernicus when he was trying to construct a heliocentric model of the solar system. He determined the order and size of the orbits of the planets he could see by measuring their respective elongations. Venus will never have an elongation greater than 47.1 degrees. But what about Mars? The angle between the Sun and Mars when seen from Earth can reach all the way to 180 degrees. The same goes for Jupiter, Saturn, Uranus, Neptune, Pluto, and so-called Planet X, though the latter four weren’t known in Copernicus’s time.
If a planet can be on the opposite side of Earth from the Sun, its orbital path around the Sun is, by definition, outside of Earth’s orbit. Located inside Earth's orbit, Venus and Mercury are called “inferior planets.” Located outside, Mars and the rest are called “superior planets.” When a superior planet is 180 degrees from the Sun from the vantage point of Earth, it is said to be at opposition. That is, a straight line is formed—Sun, Earth, planet—and that planet is in full sunlight. This is also the best time to observe such a planet.
A snapshot of the inner solar system generated by jsOrrery; the approximate line illustrates the effect of greatest elongation on Venus. Image Credit: David W. Brown
Take a look at the illustration above. At greatest elongation, a line drawn from Earth to an inferior planet will be tangent to the planet’s orbital path around the Sun. (The line will touch the orbital path, but not cross it.) Because we’re staring at this dead-on, half the planet is in full sunlight, and half is in total darkness. That’s why tonight Venus appears half-lit, and seemingly not a full circle.
So enjoy this special phase of Venus, and keep an eye on the evening sky this month for another Venus-related event—on January 31, Venus, Mars, and the sliver of a new moon will appear clustered together, forming a stunning triangle.
Editor's note: Due to an editing error, the original post referred to Venus both as a star and a planet. Of course, it's a planet.