This story was updated at 12:29 p.m. EDT.
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.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications,
and he is also an on-camera meteorologist for News 12 Westchester, New York.
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