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 was "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," Asimov wrote. "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 sky. Mercury is called an "inferior planet" because its orbit is nearer to the sun than Earth’s. Therefore, it always appears from our vantage point (as Asimov indicated) to be in the same general direction as the sun.
As legend has it
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 (48 kilometers per second) and 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 (58 million kilometers), Mercury experiences by far the greatest range of temperatures: nearly 900 degrees Fahrenheit (482 degrees Celsius) on its day side, and minus-300 degrees F (-184 degrees C) 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.
Double sunrise
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 (69 million kilometers) away. But when it arrives at its closest point to the sun (perihelion), it’s just under 29 million miles (47 million kilometers) away. So its orbital speed is appreciably greater at perihelion.
Mercury rotates on its axis three times for every two revolutions it makes around the sun. But when it arrives at perihelion (as it will on Sept. 13) Mercury’s orbital velocity will 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 (from four days before perihelion to four days after perihelion), the sun would appear to reverse its course across the sky, then double back and resume its normal track across the sky.
If our observer were located on that part of Mercury where the sun was 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!
Morning view
Back on Earth, Mercury rises before the sun all this month and is surprisingly easy to see from now through Sept. 23. 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 will be at its greatest western elongation, 18 degrees to the west of the sun, on Sept. 9, rising as dawn breaks and making this Mercury’s best morning apparition of 2004.
Mercury, like Venus, appears to go through phases much like our moon. When September began, Mercury was a slender crescent. Currently, it appears slightly less than half-illuminated, and the amount of its surface lit by the sun — and seen from Earth — will continue to increase in the days to come.
So although it will begin to turn back toward the sun’s vicinity after Sept. 9, the innermost planet will continue to brighten steadily, which should help keep it in easy view over the next couple of weeks.
Regulus rendezvous
As an added bonus this week, Mercury will be approaching the much-dimmer blue-white star Regulus in the constellation of Leo the Lion. Rising 1½ hours before the sun on the morning of Sept. 10, the Americas will see this –0.4-magnitude planet only several hours past its closest approach to the south of 1.4-magnitude Regulus. (On this astronomers' scale, smaller numbers represent brighter objects.)
From the East Coast, the distance between planet and star will be just 11 arc-minutes. That’s roughly equal to one-third of the apparent diameter of the moon. By the time Mercury and Regulus rise for the West Coast, they will have separated to 20 arc-minutes, or two-thirds of the moon’s apparent diameter.
The most striking view, however, will be from Europe. At 0519 GMT, only 3 arc-minutes, or one-tenth of the moon’s apparent diameter, will separate Mercury and Regulus. Mercury will appear nearly six times brighter than Regulus.
Speaking of the moon, it will appear to hover directly above Mercury and Regulus on the morning of the 12th, and on the following morning it will be just a delicately thin sliver, and only about 29 hours from new phase, hovering well off to the lower left of Mercury.
The speedy planet will still be easily visible as late as the Sept. 23; though nearer to the sun, it will have brightened to –1.2 magnitude — nearly as bright as Sirius, the brightest star in the night sky. Thereafter, it drops back down under the dawn horizon.