UpDate - Vol. 11, No. 21, Page 9
February 27, 1992
Next few weeks best time to study Jupiter in Leo

     Once again Jupiter appears above the eastern horizon in our
evening sky. It has been a year plus a month since this was last
true. Last year, Jupiter was in the constellation of Cancer; this
year, we find the giant planet in Leo, one constellation further
along the zodiac. Here it will shine for the next few months as the
brightest "star" of the evening sky. Venus will not be seen in the
evening hours until late July.
     The reason for Jupiter's predictable progression through the
zodiacal constellations lies in the relation of its orbital period
to that of the Earth. Jupiter takes 12 years to complete its
circuit about the sun. Consequently, after one year, when the Earth
has made exactly one circuit and is back to the same place in its
orbit, Jupiter will have moved 1/12 of a revolution further. This
puts it into the next zodiacal constellation so that it takes us an
extra month to overtake Jupiter in its new position.
     Next year, Jupiter will have moved into Virgo which does not
stand above the eastern horizon at sunset until a month later than
Leo. This interval of time is called Jupiter's synodic period. More
exactly, it is 399 days.
     Once you see Jupiter, use the sky map to find Leo in the sky.
The visibility of Leo after sunset heralds the coming spring. That
is always true; we maintain our calendar (leap years, etc.) to
match this celestial progression of seasons. But Jupiter is not
always there to help us locate the Lion. I have connected the stars
in the way most people remember the figure today: a backward
question mark (or left-handed sickle) and a right triangle.
     Illustrations in the old maps portray the Lion facing west
(right), toward Cancer. The sickle stars are the lion's mane and
the triangle is his rump. Regulus is sometimes called the lion's
heart and Denebola marks the tail.
     Regulus is the brightest star in Leo. Next is the star
Algieba. To relate the scale of the map to what you see in the sky,
hold your fist at arm's length. The size of your tightly clenched
fist should just about span the separation between Regulus and
Algieba. As you follow Jupiter's motion through the spring, you
will see it appear to move backwards toward Regulus as we pass the
planet and then, in May, resume its motion toward Virgo.
     The exact date on which Earth passes Jupiter this year is Feb.
28. Jupiter is then opposite in the sky from the sun, putting it
highest in the sky at midnight when the sun is lowest below the
horizon. The next few weeks are the optimum time for astronomy
enthusiasts to study Jupiter because it is in the sky virtually all
night long. Dedicated observers with telescopes may stay up all
night to watch Jupiter make a full rotation on its axis. This is
possible because the rotation period is only ten hours. A telescope
also reveals the larger moons of Jupiter. Their motion about the
planet becomes quite obvious during an all night "observathon."
     You may wish to wait until Leo rises higher in the sky to look
for the fainter stars. A sky brightened by the moon or by
neighbors' porch lights will wash away all but the brightest stars
on our map. But, if you can go to a dark site on a moonless night,
you will see in this field dozens of stars fainter than any
included on the map.
     Algieba (often spelled Algeiba) is shown on the map with a
different symbol. This is to indicate that there are really two
stars at that position, although it takes a telescope to see them
separately. Their closeness together and subtle difference in color
(the one star sometimes described as greenish in comparison with
its yellower companion) make Algieba one of the most popular double
stars to view with a small telescope.
                                                - Richard B. Herr
                                   Associate Professor of Physics
                                                    and Astronomy