Image by Mary Ann Sullivan
Mercury is an inferior planet. This does not mean it isn’t quite as good as the rest of the planets. “Inferior” simply means that it orbits closer to the Sun than Earth does (Venus is also an inferior planet).
Of the five naked-eye planets, Mercury is the most challenging to spot, for several reasons:
a) Mercury is a moving target. Its speedy orbit means it quickly changes position relative to the Sun. Throughout the year, it alternates between being a morning planet, visible in our eastern sky just prior to sunrise, and being an evening planet, visible in the west shortly after sunset. In either case, it’s visible for only two to three weeks at a time. If you’re not paying attention, you’ll miss it.
b) Because Mercury orbits so close to the Sun, it is sometimes hidden in the Sun’s glare. Note: It is very dangerous to attempt to locate Mercury while the Sun is above the horizon. You could unintentionally look directly at the Sun and permanently damage your eyes. Don’t try it!
c) Because Mercury hangs tight with the Sun, it typically sets soon after the Sun sets or rises shortly before the Sun rises. This limits your windows of opportunity for spotting it.
Now for the good news. We are currently enjoying the best evening apparition of planet Mercury in 2008. This observing week, Mercury reaches its greatest elongation east. This means that, from our perspective on Earth, Mercury and the Sun are about as far apart as they can get during an evening apparition. This separation helps with both (b) and (c) above.
Here’s your chance to bag the smallest and swiftest for your planetary trophy case!
1) Find an observing spot with an unobstructed view of the western sky, i.e. no tall buildings, trees, big hills, etc. Get there just before sunset, so you can make note of where on the horizon the Sun sets. Remember that spot, using a natural or manmade visual marker. Never stare directly at the Sun!
2) Now find the crescent moon in the sky, because you can use it to locate Mercury. As I’ve mentioned in a number of previous posts, the Moon and the planets can be found along the ecliptic. The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. So, if you know where the Sun set, and you see the Moon in the sky, and you draw an imaginary line between the two, you can approximate the ecliptic. Along that line is where you’ll find bright planet Mercury.
3) On the evening of Friday, May 9th, Mercury will be a bit lower than halfway from the Moon to the spot where the Sun sets. Depending on your eyesight and your ability to spot star-like objects while the sky is still illuminated by twilight, it might take up to half an hour after sunset before you spot Mercury. By then, it should be dark enough for anyone to spot it. Don’t worry, you’ll have plenty of time to find it; Mercury won’t set until about two hours after sunset. Obviously, if there are any clouds in the western sky, Mercury could be obscured. If so, try again another night this week.
4) About half an hour after sunset, Mercury’s altitude, that is, its distance above the horizon, will be around the height of your fist. Make a fist and hold it at arm’s length in front of you, with your thumb at the top. Place the bottom of your fist at the horizon. Look for a bright star-like object at or slightly above the top of your fist. Mercury will be the second brightest “star” in the western sky. Only Sirius the Dog Star, low in the southwest, will be brighter. Find it yet? Congratulations on your planetary conquest!
By the way, the bright star just below and to the left of Mercury is Aldebaran, aka the Eye of the Bull, in the constellation Taurus.
5) At the same time each night after the 9th, the Moon will be a bit higher in the sky, but Mercury will be around the same altitude above the western horizon. You’ll simply need to draw a longer imaginary line to connect the Moon and the sunset point.
6) Mercury is known as the “Pink Planet.” I looked askance at the first person to tell me that, but darned if it doesn’t look pink or peach every time I observe it. Do you see any color? Compare Mercury to stars around it. Try comparing it to bright Sirius, which is blue-white in color. Sometimes comparison will help you see its subtle pink or peach hue. Of course, if you are of the male persuasion, you may--like my significant other--relentlessly dispute that pink and peach are even different colors.