Tonight, if it’s clear at your place, you can look up and see a thick crescent moon hanging below the planet Mars. Mars is our only bright evening planet right now. The others — Venus and Jupiter — are visible on the other side of the sky at dawn. The lunar crescent and Red Planet will be just 5° or three fingers held together at arm’s length apart tonight. When two bodies line up, one above or below the other, we say they’re in conjunction. In a week, it will be Venus and Jupiter’s turn — more on that in the coming days.
Conjunctions concentrate the sky’s bright jewels in a small area and make for beautiful sights. They occur because all the planets and the moon follow along the same “highway” in the sky called the ecliptic, an invisible circle running through the zodiac constellations that represents the plane of the solar system. As each planet orbits the sun (or Earth in the case of the moon) at its own individual speed, one occasionally passes in front of another. They appear close together in the sky because we see them along the same line of sight not because they’re physically close.
Conjunctions aren’t reserved for earthlings. They occur at other planets, too. Standing on Mars, you can watch the pale blue Earth line up with Jupiter or Venus. If you’re watching from Jupiter you can watch conjunctions of Earth and Mars. Pretty cool. When two bodies pass so close to one another that they overlap or one covers the other, we call that an occultation.
I’ve included a few different views of how things look elsewhere just to tickle the imagination. Think of all the alignments that must occur across our solar system in a single day given all the pieces nature has a to play with: a sun, Earth’s moon, 8 planets, 171 moons orbiting those planets and 8 moons orbiting the smaller dwarf planets. Anything’s possible!
A NASA camera aboard the Deep Space Climate Observatory (DSCOVR) captured a unique view of the Moon on July 16, 2015 as it passed between the spacecraft and Earth in a partial occultation or transit. Two things stand out in this photo. First, we see the far side of the moon fully illuminated just like the Earth. Second, you can really get a good sense of how dark the moon really is compared to the Earth. The colors and tones are the true ones. There’s a slight color artifact around the moon’s edge because it moved during the three exposures required to take this photo. Color photos taken by spacecraft are compiled by taking three separate exposures through red, green and blue filters and then combining them to make a color image. DSCOVR EPIC team