If you have a set of binoculars or a telescope, watch for Venus’s thinning crescent over the next couple weeks.
Seeing the phases of the Moon is a familiar experience: Waxing crescents, first quarters, and full moons are known to everyone, even those who may not know the precise terminology. So it’s one of the delights of astronomy to learn that the Moon isn’t the only solar system body to exhibit phases — the planets Mercury and Venus do, too.
For these planets, the mechanics behind the phases are somewhat different, as is the visual experience. In this article, we’ll specifically look at the phases of Venus, because the visual impact of the phases are more impressive and it’s a much easier object to observe. But keep in mind that the concept applies to Mercury too, it’s just a more challenging target.
Like the Moon — But Different
First, let’s start with what we already know: the Moon. As the Moon travels through its monthly orbit around Earth, we see different percentages of its surface illuminated by the Sun. To be sure, half of the Moon is always illuminated, but our perspective relative to that half changes over time. When the Moon is full, we’re seeing the lit 50% straight on. When it’s a crescent, it’s mostly the other side of the Moon that’s in sunlight. Also, no matter what the phase, the Moon appears roughly the same size in the sky, because it’s always about the same distance away from Earth.
With Venus, there are a couple of twists. Venus orbits the Sun, circling interior to Earth every 225. In its smaller orbit, Venus speeds around the Sun more quickly than Earth does. This means that Venus is sometimes relatively close to Earth; other times it’s on the other side of the Sun. It’s this change in relative positions that causes the phases of Venus.
When Venus is on the other side of the Sun from Earth (technically a superior conjunction), it is being lit straight on from our perspective — the equivalent of a full Moon. But Venus is also at its farthest distance from Earth at this time. So in a telescope, Venus appears like a fully illuminated but comparatively tiny disk. As it comes closer to us, the apparent size increases, and the phase changes. By the time Venus appears “half” lit (the equivalent of a first- or third-quarter Moon), it has grown considerably in the telescope eyepiece.
Probably the most exciting phase to watch is the crescent, which appears when Venus and Earth make their closest approach to each other, at inferior conjunction. (This year, Venus’s inferior conjunction occurs on June 3rd.) For weeks before and afterward, Venus appears much larger than at its fully lit phase, and its thin crescent provides a visually impressive telescope target. This time is also when Venus is at its brightest in our sky, maxing out at around –4.7 magnitude. The phases of Venus are opposite those of the Moon in the sense that the Moon reflects maximum light during its full phase, while Venus does so as a crescent.
A Long-term Viewing Project
The Moon completes a cycle in about a month (indeed, the word “month” is derived from “moon”), but watching a full cycle of Venus phases is a longterm observing project that takes several months to complete. When Venus is visible in the evenings, you can use it to begin your night’s astronomy, viewing the phase at dusk while you wait for the night to darken for deep-sky observations.
And don’t be disappointed that only a small amount of Venus’s globe is illuminated when it’s closest to us. The bright, thick clouds prevent us from viewing any surface details anyway, and cloud texture is rare and subtle at best. So just enjoy the sight of the large crescent!
A note on safety: Because Venus is always somewhat near the Sun, it's wise to wait until the Sun is below the horizon (either before sunrise or after sunset) before searching for Venus with a telescope or binoculars. By doing this, you eliminate any possibility of the Sun accidentally entering your instrument's field of view.
A project like this puts you in good astronomical company: In the 1600s, Galileo, too, watched the phases of Venus with his early telescopes. His observations were a major piece of evidence against the then-accepted geocentric (Earth-centered) views of the solar system.
The Rules Only Apply Here
Other than Mercury, Venus, and the Moon, you won’t see any other solar system bodies displaying crescents from our vantage point. That's because all the other planets and moons lie beyond Earth’s orbit, and so they're always lit straight on from our perspective (though attentive eyes may see is an occasional gibbous appearance from Mars).
However, if you travel out into the solar system, all bets are off! In 1973, the unmanned Pioneer 10 probe crossed Jupiter's orbit and was able to photograph the giant planet’s crescent phase for the first time — a view impossible to witness from Earth. Many other outer solar system bodies have been photographed this way in the decades since. From the right location, any planet or moon will display phases — even Earth!