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What is an
Arc-Second?
On August 27, Mars will make its closest approach to earth. On that
date, it will subtend an angle of 25.1 arc-seconds, thus making surface
features visible in most backyard telescopes.
Arc-seconds is an measure of angle against the sky. There are 360
degrees in a circle. In each degree of a circle, we measure 60 minutes
of angle, called an arc minute. There are also 60 seconds in a minute.
Thus, an arc second is 1/60 of an arc minute, or 1/60 of 1/60 of a
degree. Very narrow.
A measurement in arc-seconds gives an idea of how wide an object will
appear in the eyepiece of a telescope.
For example, the moon subtends a diameter of approximately one half
degree in the sky, which is about 30 arc minutes. That is about 1,800
arc-seconds. At 25.1 arc-seconds in diameter, Mars is still relatively
tiny, but still much larger than usual, not much smaller than the size
of Jupiter in the eyepiece.
NASA Photo Mars
2001.
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Opposition Timetable
In January, Mars is not far to the right of Venus in the first
few weeks of 2003. The rival of Mars in color, Antares is brighter than
Mars this month. In February, Mars and Venus are still together
in the dawn sky, but continue to separate. At month's end, they are well
to either side of the Teapot in Sagittarius. In March, Mars
rises several hours before dawn and makes a fascinating passage through
the deep sky wonders of Sagittarius. In April, Mars rises
several hours before dawn and is fairly high in the south-southeast as
morning twilight glows. It is still only 8" to 9" wide in telescopes,
hardly large enough to show any surface details. In May, Mars
still rises well after midnight, but it is now the brightest object in
the sky, a negative-magnitude orange fire. A telescope reveals Mars
growing to more than 10" across. It is now possible to see a few details
on steady nights. Spring begins in the southern hemisphere of Mars on
May 5, so we should already be able to see the polar ice-cap. In
June, Mars finally starts to rise as early as midnight, and has
brightened to a spectacular magnitude -1.5. The disk now swells from 12"
to 16". By now, grey or seemingly green surface markings should be
visible in medium-sized telescopes. In July, Mars doubles in
brightness, its orange flame now burning at -2.3. Mars begins retrograde
motion relative to the stars July 29-30. The relative size of the planet
is now 16" to 22"—the biggest it has appeared since 1988. The southern
half of Mars is becoming ever more tilted toward Earth and the rapidly
melting polar ice-cap is a fascinating sight. On the morning of July 17,
viewers in North America can see Mars perched on the upper edge of the
Moon; there will be a spectacular occultation of Mars for viewers in the
extreme southeastern US. In August, Mars, located in the
constellation Aquarius, makes its closest approach to Earth on August
27, with an apparent diameter of 25.1" and a magnitude of -2.9. The
planet reaches perihelion on August 30, only two days after opposition.
Observers should see intricate details on the Martian surface, unless
they are obscured by dust storms, but the southern polar ice-cap will
now be a tiny speck as the Martian summer approaches. For observers in
northern latitudes, Mars will be only 34 degrees above the southern
horizon. If you have a fairly large telescope, try to catch a glimpse of
the Martian moons, Phobos and Deimos. In September, Mars still
holds center stage at magnitude -2.9 and diameter 25". Earth now begins
to pull rapidly away from the planet and Mars will begin to lose
brightness. By month's end, magnitude will have decreased to -2.2 and
diameter to 22". But Mars now reaches its highest point before midnight,
making it convenient for observations, with a higher likelihood of
steady skies. In October, Mars is still a stunning sight,
although it will fade from -2.2 to the brightness of Sirius by month's
end. The diameter of the planet will also shrink from 22" to 15". Mars
now does not set until well after midnight. In November, Mars
remains a bright object, but its diameter has dwindled from 15" to 11".
It is still big enough to show some interesting surface markings on
steady nights. In December, Mars is still the most prominent
object in the sky, shining at about 0 magnitude. Unfortunately, the
planet is only 40% as wide as it was in August and shows few surface
markings in medium-sized telescopes. |
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