See Mercury, the elusive planet

Pinnacle: May 14
Mercury, like Venus, appears to go through phases like the moon. Right now, Mercury is a nearly full disk, which is why it starts off appearing so bright. Mercury reaches its greatest elongation, 22 degrees to the east of the sun, on May 14. Shining at magnitude +0.4 (as bright as the star Procyon in Canis Minor), it will set nearly two hours after the sun. By the time it arrives at its greatest elongation, it will appear roughly half-illuminated and the amount of its surface illuminated by the sun will continue to decrease in the days to come. So when it begins to turn back toward the sun's vicinity after May 14, it will fade at a rather rapid pace.

In fact, on the evening of May 22, Mercury's brightness will have dropped to magnitude +1.7; only 1/20 as bright as it was on April 23. In telescopes it will appear as a narrowing crescent phase. This, in all likelihood will be your last view of it, for the combination of its lowering altitude, plus its descent into the brighter sunset glow will finally render Mercury invisible by the final week of May.

Orbital peregrinations
In old Roman legends, Mercury was the swift-footed messenger of the gods. The planet is well named for it is the closest planet to the sun (at an average distance of 36 million miles) and the swiftest of the sun's family, averaging about 30 miles per second; making its yearly journey in only 88 Earth days. Interestingly, the time it takes Mercury to rotate once on its axis is 59 days, so that all parts of its surface experiences periods of intense heat and extreme cold. But we cannot say that a solar day on Mercury lasts 59 days because its highly eccentric orbit causes an effect known as a 3:2 spin–orbit resonance. As a consequence, because Mercury rotates three times for every two times it revolves around the sun, the time it takes from one sunrise to the next is equal to about 176 days.

Space.com/StarryNight.com May 14 will probably be your last chance to see Mercury. It will dim rapidly from here on in, as it swings around between us and the Sun, showing us its dark side.

So for 88 days, the sun is constantly above the horizon and surface temperatures soar to nearly 900 degrees Fahrenheit. Daytime temperatures are high enough to melt zinc and tin. Then comes 88 days of darkness, with surface temperatures plummeting to -300 degrees F — the greatest range of temperatures of any planet.

In fact, on some parts of Mercury's surface, it is even possible to see the sun briefly reverse its course in the sky. This happens around the time that Mercury arrives at that part of its orbit which places it closest to the sun (called perihelion), where it's orbital speed can briefly exceed its rotational speed.

At sunrise, this strange effect would cause the sun to appear to rise above the horizon ... then slow to a halt ... then reverse direction and drop back below the horizon. At sunset, the sun would drop below the horizon, then pop back up into view before resuming its normal course, ultimately setting again for a second time within a week.

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