Planet collectors should look west at dusk to get a rare glimpse of the hot red rock.
Have you ever seen Mercury, the smallest and fastest-moving planet in our solar system? Since it orbits close to the sun, it's usually so low on the horizon just before sunrise and after sunset that it’s impossible to see. However, that changes briefly this week when Mercury appears to be at one of its furthest points from the sun, as seen from Earth.
How to See Mercury in the Night Sky
On Wednesday, Mercury will be 18 degrees from the sun, so high enough above the western horizon to be seen relatively easily. Astronomers call this Mercury's "greatest eastern elongation." As viewed from the northern hemisphere, it will be visible (clear skies allowing) due west as a bright red dot immediately above where the sun has just set. To see it properly a pair of binoculars will help, but don't even think about using them until the entire disk of the sun has sunk below the horizon.
When to See Mercury in the Night Sky
If you need an excuse to see a sunset this week, Mercury has you covered. Just after the sun dips below the western horizon each evening between Tuesday and Friday this week will be a great time to go Mercury-spotting. Sunset this week occurs between 5:43-5:47 p.m. EST in New York, and 5:46-5:49 p.m. PST in Los Angeles. Make sure you're in place around then, because Mercury will quickly sink as the sky darkens. However, it's worth hanging around for 30 minutes or so since it may become slightly brighter in the quickening twilight.
Unlike the outer planets, which take many years to complete an orbit, Mercury takes just 88 days to complete an orbit of the sun. The next time Mercury will be briefly visible will be shortly before sunrise in early April 2019.
How to See Mercury and a 'False dusk'
If you're in a dark sky destination, such as a Dark Sky Park or anywhere rural about 40 miles from the nearest town, Wednesday is also a great night to see something called zodiacal light. Also called a "false dusk," this is the weird spectacle of sunlight shining off dust in the solar system. It's visible as a triangle of light, and it just so happens that this time of year is perfect for spotting it when there's no moonlight. That's the case this week. It does require absolute darkness, but you may just be able to watch Mercury sink as false dusk rises.
What Is the Transit of Mercury?
2019 is a big year for Mercury-spotting. A rare event that occurs only 13 times each century, a Transit of Mercury is when the tiny planet appears to cross the sun's disc as seen from Earth during the day. The event takes only about five hours, but next occurs on Monday, Nov. 11, 2019. However, not everyone on Earth will be able to observe it. As well as needing a telescope protected by solar filters, only those in North America, South America, and Africa will get a good view. Although it's an early morning event in North America, it's close to sunrise and therefore to the horizon, so astronomers will likely be traveling to Chile's Atacama Desert to more easily view the rare spectacle, which will be much higher in the sky. There’s also a higher chance of clear skies.
A Transit of Mercury is a great excuse to travel, though it's best to aim for a public observatory and let them handle the telescope and the must-have solar filters. If you miss it, you’re in for a long wait because the next Transit of Mercury is in 2032.