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 vantagepoint (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 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 Celsius) on its day side and minus 300 degrees Fahrenheit
(-184 Celsius) 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 September 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 (fours 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 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!
Morning view
Back on Earth, Mercury rises
before the Sun all of 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's 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 September 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 of the
United States, 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 of the United States, 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 5:19 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 - nearly as bright as Sirius, the brightest star in
the night sky. Thereafter, it drops back down under the dawn horizon.
|
|
Map
Mercury from Home!
Starry
Night software brings the universe to your desktop. Map the sky from
your location, or just sit back and let the cosmos come to you.
LEARN
MORE!
|
|
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.
|
DEFINITIONS
|
|
Degrees
measure apparent sizes of objects or distances in the sky, as seen from
our vantage point. The Moon is one-half degree in width. The width of
your fist held at arm's length is about 10 degrees.
Magnitude
is the standard by which astronomers measure the apparent brightness
of objects that appear in the sky. The lower the number, the brighter
the object. The brightest stars in the sky are categorized as zero or
first magnitude. Negative magnitudes are reserved for the most brilliant
objects: the brightest star is Sirius (-1.4); the full Moon is -12.7;
the Sun is -26.7. The faintest stars visible under dark skies are around
+6.
|
advertisement
