Image: Mercury
On the morning of October 26, midnorthern viewers can look for Mercury and the moon rising before the sun, less than a handsbreadth apart.
updated 10/17/2008 4:58:48 PM ET 2008-10-17T20:58:48

If there ever was a planet that has gotten a bad rap for its inability to be readily observed, it would have to be Mercury, known in some circles as the "elusive planet."

In his book "The Solar System and Back" (Doubleday, 1970), famed science writer Isaac Asimov (1920-1992) argued that the planet Mercury:

". . . is hardly ever visible when it is truly dark. Mercury . . . will be seen only near the horizon in dawn or twilight, amid haze and sun glare. I suspect, in fact, that many people today (when the horizon is dirtier and the sky much hazier with the glare of artificial light than it was in centuries past) have never seen Mercury."

Nonetheless, during these next three weeks we will be presented with an excellent opportunity to view Mercury in the early morning dawn sky. Mercury is called an "inferior planet" because its orbit is nearer to the sun than the Earth's. Therefore, it always appears from our vantage point (as Asimov indicated) to be in the same general direction as the sun.

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 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 experience periods of intense heat and extreme cold. Although its mean distance from the sun is only 36 million miles, Mercury experiences by far the greatest range of temperatures: nearly 900 degrees Fahrenheit on its day side, -300 degrees on its night side.

In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the sun and then the other. Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning. It is said that Pythagoras, about the fifth century B.C., pointed out that they were one and the same.

Sunrise, sunset
Mercury possesses the most eccentric orbit of any planet except Pluto. At its farthest distance from the sun (aphelion), it lies about 43 million miles away. But when it arrives at its closest point to the sun (perihelion) it's just less than 29 million miles away. So its angular velocity through space is appreciably greater at perihelion. Interestingly, Mercury rotates on its axis three times for every two revolutions it makes around the sun. But at perihelion (Oct. 17) Mercury's orbital velocity will briefly exceed its rotational speed.

As a consequence, a hypothetical observer standing on Mercury would see a sight unique in our entire solar system. Over the course of eight days (fours days before perihelion to four days after perihelion), the sun will appear to reverse its course across the sky, then double back and resume its normal track across the sky. If an astronaut were located on that part of Mercury where the sun were to rise around the time of perihelion, the sun would appear to partially come up above the eastern horizon, pause and then drop back below the horizon, followed in rapid succession by a second sunrise!

Mercury rises before the sun all of this month and is surprisingly easy to see from now through Nov. 5. All you have to do is just look low above the eastern horizon during morning twilight, from about 30 to 45 minutes before sunrise for a bright yellowish-orange "star."

Mercury in the morning
Mercury will be at its greatest western elongation, 18 degrees to the west of the sun, on Oct. 22, rising as dawn breaks. Mercury, like Venus, appears to go through phases like the moon. Shortly after passing inferior conjunction on Oct. 6, Mercury was just a slender crescent. Currently, it appears about one-third illuminated, but the amount of its surface illuminated by the sun will continue to increase in the days to come. So although it will begin to turn back toward the sun's vicinity after Oct. 22, it will continue to brighten steadily, which should help keep it in easy view over the following couple of weeks.

Rendezvous with the moon and a star
Helping to aid in identifying Mercury will be a lovely crescent moon. Early on the morning of Oct. 26, at about an hour before sunrise, you'll find the moon low in the east-southeast sky and Mercury will appear as a bright star-like object well below and to the moon's left.

On the following morning (Oct. 27), it will be just a delicately thin sliver, and only about 38 hours from new phase, hovering well off to the lower right of Mercury. Then during the mornings of Oct. 30 and 31, Mercury will slide above and to the left of the bright blue star, Spica, in the constellation of Virgo.

The speedy planet will still be easily visible as late as Nov. 5; though appearing nearer to the sun's vicinity in the sky, it will have brightened to magnitude -0.9. That's brighter than the star Canopus and second in brightness to Sirius, the brightest star in the night sky. Thereafter, it drops back down to invisibility, under the dawn horizon.

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