Look for the planet low in the western sky just after sunset.
Have you ever seen the planet Mercury with the naked eye? If not, tonight could be the night. Look south after dusk this month and the red planet Mars remains a very bright object having been visible all summer long. Venus is another planet that is easy to see and is currently shining brightly just before dawn in the East. However, since Mercury is so close to the sun, it's almost always lost in its glare. That changes just occasionally, and tonight is one of those rare times.
When to see the planet Mercury
It's only possible to see Mercury from Earth during twilight just before dawn in the east, or just after dusk in the west, and it’s always close to the horizon. That can make it difficult to observe, and it does require both effort and luck. The tiny planet, which orbits the Sun every 88 days, is easiest to see from Earth when it appears furthest from the sun, and thus higher above the horizon. That happens every 116 days, at what astronomers call its 'greatest western elongation' when it's visible just before sunrise, or 'greatest eastern elongation' when it can be seen just after sunset. Tonight, Mercury reaches its greatest eastern elongation, so can be seen in the south-west sky between sunset and Mercury-set. In New York, that’s between 4:46 p.m. EST and 5:54 p.m. EST, and in Los Angeles, between 4:55 p.m. PST and 6:01 p.m. PST.
How to see the planet Mercury
As well as looking just after dusk, finding Mercury in the night sky is challenging because it quickly sinks. So you need to be up somewhere high with a good view of the horizon. The narrow window for seeing Mercury can also be made more difficult by clouds, which are more common on the horizon. It may be the smallest planet in the solar system, but Mercury is truly tiny. In fact, it's smaller than one of Jupiter's moons, Ganymede, which is easier to see in the night sky than Mercury. However, there are a couple of bright objects near to Mercury that will help you find it. Close by on Mercury's left will be the bright red star Antares, while further to its right will be the planet Jupiter.
You should use binoculars to give yourself the best chance of finding Mercury (it looks like a small orange-yellow light), but be very careful not to use binoculars while even a tiny section of the sun is above the horizon. If you accidentally look at the sun through binoculars or a telescope it can cause damage to your eyes.
Are there any spacecraft at or near Mercury?
No, and it's a very poorly understood planet with an odd chemical composition. However, a couple weeks ago the European and Japanese space agencies launched BepiColumbo, a spacecraft containing two orbiters that will study the planet’s interior, geology, atmosphere and how it interacts with the solar wind from the sun. It will take seven years to get there, via flybys of Earth, Venus and Mercury itself as it attempts to speed up and slow down enough to go into orbit around the tiny planet.
When is the next transit of Mercury?
It's coming up real soon. Although it's really hard to see Mercury in the night sky, just occasionally the planet crosses the disc of the sun, so a silhouette of it can be seen crossing the sun’s disk during the day. This happens 13 times each century, and always in either May or November. Although you need a telescope to see it happen, and one with special solar filters so that you don't damage your eyes, it will be possible to see Mercury cross at least part of the sun’s disk from North America on Nov. 11, 2019. The transit will take just over five hours. The last time a transit of Mercury occurred was in May 2016, but the next is not until Nov. 13, 2032.