Find Mercury near Venus this week and into next

Jan

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Marveling at Mercury

Have you ever seen Mercury? Not the metal found in thermometers; the actual planet. It is surprisingly bright, but many people don't know when or where to look. This month offers the perfect time to try and spot this elusive solar system speedster.

Mercury is the innermost planet of the solar system. As such, we refer to both it and Venus as "inferior" planets - that is to say, they orbit the Sun inside of Earth's orbit (the other planets are called "superior" planets). Because of this location in our solar system, these planets never cross the sky, and they never reach opposition with Earth. Rather, the best time to see them is when they obtain the position of "greatest elongation." Yet Mercury is somewhat elusive. It orbits the Sun in just 88 days - less than a full season on Earth. Despite shining at over magnitude 1 much of the time when it is brightest, it is forever stuck in the twilight glow of the Sun. One never sees it under a truly dark sky, so observers really HAVE to be looking for it in order to not only see it, but positively identify it.

So if it is constantly stuck in twilight glow, and moves quickly, how do we find it? This greatest elongation for Mercury, there's a veritable lighthouse of a beacon nearby: The brilliant Venus. Mercury actually reaches greatest western elongation on January 14, but it sits within a degree or two of Venus for several days before and after. So if you have the opportunity due to clear skies to the southwest, start looking now.

Before going too far, a bit about measuring distance in the sky. You can easily measure how far objects are from each other by just using your hands. Your fist, held at arm's length, is about 10 degrees across from one side to the other. And your pinky at arm's length is approximately one degree of sky. Yes, this really works for most everyone! The key is, it is close enough to work for most people, and that's what matters.

So how can you find the two and identify them? There are quite a few first magnitude stars in that part of the sky as the Sun is going down. About 30 to 45 minutes after sunset, to the northwest, 0 magnitude Vega is close to setting, and first magnitude Deneb is above it. The third point of the Summer Triangle, Altair, will be nearly due west for most United States observers, and is about 35 degrees from Vega. Now look a similar distance towards the west-southwest. Venus will be shining brighter than any of those stars - you can easily determine which point it is, because splits the difference between Altair in the west, and first magnitude Fomalhaut in the south-southwest. And reddish Mars at magnitude 1.3 is higher up in the sky, partway between Venus and Fomalhaut.

Now look carefully at the brilliant, cloud-covered planet of Venus. Just to the right for northern hemisphere observers, a point similar in brightness to Vega will be there. Mercury will be shining a magnitude zero - brighter than Altair or Fomalhaut, but dimmer than Venus. And on the 12-th and 13-th, the planet is at or less than one degree from Venus. That's about that "one pinky width at arm's length" distance.

Watch the Mercury/Venus animated gif above. On the 14-th, watch what happens; there's now separation starting to really occur between the two. This is the date of Mercury's greatest elongation, that "furthest point away from the Sun from our perspective" location in the sky. But that doesn't mean it stops being visible right away. It will still take several days for it to sink back towards the Sun. The difference now is that the separation between it and Venus becomes even greater.

By the 15-th, the two are nearly 2 degrees apart; by the 16-th, just over 2 degrees. But that is STILL an easily-observed distance to locate the innermost, fast-moving planet.

On the 17th and 18-th, Mercury slips to about 3 degrees away from Venus, and by the 19-th, it is really moving. At four degrees now - and moving more towards the Sun, the observer will need to follow a bit of a bent line from Mars through Venus and then carefully measure to see Mercury. Remember, the closer it moves the Sun, the more twilight glow engulfs the planets.

By the 20-th, the innermost rock from the Sun is over 5 degrees from Venus - half your "10 degrees" fist-width measuring tool. And the planet dims in brigthness too. Watch the planet as it sinks over the next few days if possible, but hopefully you'll have clear skies at least from the 12-th through the 20-th to try and spot the Messenger planet at least once or twice.

On the 21-st and 22-nd, the Moon joins the planetary action, though this is a VERY slender, waxing crescent Moon. Binoculars are almost guaranteed to be needed to see the thin sliver of lunar light - BUT! - the nearby planets should make finding our large natural satellite's glow easier.

This conjunction with Venus offers one of the best opportunities in a long time to be able to spy Mercury, so look now - many astronomers never even have! So look now - it won't take long to find it, and even if it's cold where you are, remember that the Sun-facing side of Mercury is nice and warm at 800 Fahrenheit... but also a LOT colder on the unlit side, dropping to an atom-slowing 280 degrees Fahrenheit.









To see more in the night sky with a small telescope, check out the First Light Guides right here at Eyes on the Sky. They will walk you through how to find great objects to view through a small telescope 60mm or larger from most anywhere.