The Messenger

April 18, 2008

Creative Commons License photo credit: Wolfgang Staudt

Beginning at the end of this month, and for most of May, we have our best opportunity of the year to observe the most elusive planet, Mercury

Face west at dusk and look over the point of sunset to see it.  The best time is around May 12, although the planet is visible for two weeks to either side of that.  By Memorial Day, Mercury is once again lost in the Sun’s glare. 

Ancient Greeks associated the planet with Hermes, the swiftest of the Olympian gods who served as their messenger.  Ancient Romans then associated their god of trade, Mercury, with the Greek Hermes.  This is because Mercury moves among the stars faster than all other planets, but possibly also because it appears in the sky only briefly, for only a few weeks at a time.  Factor in some cloudy nights, and Mercury might seem to be ‘here today, gone tomorrow’.

Creative Commons License photo credit: garryknight

Why is Mercury so hard to see?  Because Mercury is the closest planet to the Sun.  Planets like Mars, Jupiter, or Saturn have orbits outside ours.  When these planets are on the far side of the Sun from us, they appear in the Sun’s general direction, but if they’re on our side of the Sun, they are ‘behind’ us and we see them opposite the Sun in the sky.  At opposition, a planet is in the sky all night long. 

Mercury’s orbit, on the other hand, is entirely in ‘front’ of us; Mercury can’t come to opposition.  Whether it’s on our side of the Sun or on the far side, Mercury is always generally in the Sun’s direction, and it’s rare to have Mercury above the horizon when the Sun is not also up. 

When Mercury is directly on the far side of the Sun, it’s of course completely impossible to see. Mercury, however, orbits faster than any other planet, so Mercury eventually emerges from behind the Sun and appears to the Sun’s ‘left’.  Mercury now remains in the sky a few minutes after sunset; this is the best time to observe it. 

Then, Mercury comes around to our side of the Sun.  Of course, we’re moving too, so this takes a bit longer than if we were still, but Mercury is faster, so it always catches up and ‘laps’ the Earth like a faster runner on the inside track.  This brings Mercury back into alignment with the Sun, making it invisible once again.  Mercury quickly pulls ‘ahead’ of Earth on its faster orbit and begins to appear ‘right’ of the Sun from our perspective. It now rises shortly before the Sun and is visible to early risers. 

Finally, Mercury moves around to the far side of the Sun once more, and the cycle repeats.

Today, April 16, 2008, Mercury is in line with the Sun and us, on the far side of the Sun.  We therefore can’t see it just yet.  For the rest of April, as it moves from behind the Sun, we’ll see Mercury gradually emerge from the Sun’s glare and appear in the evening sky just over the point of sunset.  Begin looking next weekend. 

By mid-May, Mercury appears as far to the ‘left’ of the Sun as it’s going to get; it’s about to come around to our side.  This is the moment of greatest ‘elongation’, or greatest apparent separation from the Sun.  This is therefore the best time to observe Mercury. 

Creative Commons License photo credit: euthman

Mercury takes until June 7 to come all the way to our side of the Sun and line up with us.  Beginning in late May, then, we’ll see Mercury move back towards the Sun and again be lost in the Sun’s glare by the end of May. 

Also, we see Mercury’s day side when it is on the far side of the Sun, but Mercury turns more of its night side to us as it approaches Earth.  Therefore, Mercury shows distinct phases when viewed through a telescope. Also, Mercury is slightly brighter when it first emerges into the evening sky than when it’s about to leave. 

Surf to ‘Solar System Live’ to observe these alignments for yourself.

Why is this apparition the best of the year?  Because of the geometry of our orbit and Mercury’s.  Our solar system is fairly flat; all planets orbit nearly (although not exactly) in the same plane.  It turns out that the angle between this plane and the horizon is steep on winter and spring evenings and shallow on summer and fall evenings.  At these times, almost all of the apparent separation from the Sun translates into height above the horizon, making Mercury easier to see. 

The reverse is true in the morning; the angle is steeper on summer and fall mornings than in winter or spring.  If Mercury comes out from behind the Sun into the evening sky between January and May, it’s easier for us to see.  When this happens between July and November, it’s better for the Southern Hemisphere. (That’s winter and spring down there). 

Further, planetary orbits aren’t perfect circles, but ellipses which are almost circles.  On a scale of 0 to 1, however, the eccentricity (out-of-roundness) of Earth’s orbit is only about 1/60, and that of Venus is about 1/150.  Mercury has a much more significant eccentricity of about 1/5.  Since the distance from Mercury to the Sun varies significantly, it sometimes emerges farther from the Sun into our sky that at other times.  Greatest elongation this time is 21 degrees (your fist at arms length blocks 10 degrees).  Mercury is slightly higher in the sky this May than the last time it entered the evening sky in January. 

The next apparition of Mercury which is almost this good is in October 2008–in the morning just before dawn.  Step outside next month, then, and enjoy a glimpse of the Messenger before he takes off on his next errand.

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

One response to “The Messenger”

  1. Nice level of information here. There is so much data around about this subject that sometimes you cannot see the wood for the trees but you have pitched this at just the right level so that the lay person can understand – thank you!

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