Nonetheless, the next two weeks present an excellent opportunity to view this elusive "star" of the early morning dawn sky.
Inferior and swift
Mercury is called an "inferior planet" because its orbit is nearer to the Sun than the Earths. 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 Suns family, averaging about 30 miles per second and making its yearly journey in only 88 Earth days. Mercury rotates once on its axis every 59 Earth-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, from a scorching 900 degrees Fahrenheit on its day side to minus 300 degrees on the night side (a range of -184 to +482 degrees Celsius).
Mercury's other name
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.
Mercury rises before the Sun all of this month and is surprisingly easy to see from now through Oct. 27. All you have to do is just look along the eastern horizon during morning twilight, from about 30 to 45 minutes before sunrise for a bright yellowish-orange point of light.
As an added bonus this week, the much-dimmer Mars will be nearby. These two planets were actually closest to each other on Oct. 10, when Mars hovered just above and to Mercurys right. Each night, Mercury and Mars will continue to slowly separate; Mercury continuing to appear lower down and to the left of Mars.
Even brighter planets
Higher in the morning sky and unmistakably bright is Jupiter. Still higher, in fact almost directly overhead, is Saturn, which slightly outshines several bright stars overhead at dawn. Saturn can be found by tracing a line that extends from Mercury and through Jupiter. Along that line, Saturn is almost exactly as far from Jupiter as Jupiter is from Mercury.
Mercury reaches its "greatest elongation," 18-degrees to the west of the Sun, on Oct. 13. This is when it appears highest in our morning sky. Shining at magnitude 0.3, the inner planet rises as dawn breaks making this Mercurys best morning apparition of 2002. Magnitudes are used by astronomers to denote a celestial object's brightness, as seen from Earth. The brightest objects shine at negative magnitudes.
Mercury, like Venus and the Moon, goes through phases. When October began, Mercury was a skinny crescent. Currently, its appears roughly half-illuminated and 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 Suns vicinity after Oct. 13, it will continue to brighten steadily as its reflecting area grows. This should help keep it in easy view over the next couple of weeks.
By Oct. 27, the elusive planet will have brightened to magnitude 1.0, surpassing the brilliance of every star in the sky with the exception of Sirius (the brightest of all stars). Also on this date Mercury will be passing to the north of the bluish first magnitude star Spica, in Virgo. Spica, however, will be only appear about 1/6 as bright as Mercury, so youll probably need binoculars to spot it.
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