SKYLIGHTS
Astronomy news for the week starting Friday, June 4, 2000.
The next Skylights will appear as usual, on Friday, June 9. The
Moon grows through its waxing crescent phase this week, beginning
far in the northwest near the setting Sun and moving both to the
east and south against the background stars. It passes first
quarter on Thursday, June 8. Be sure to note "earthlight," the
illumination of the nighttime side of the Moon by the brilliant
Earth. Just as the Moon goes through phases as seen from Earth, so
does the Earth as seen from the Moon, but in reverse. When we see
the Moon as a waxing crescent, an imaginary "lunarian" would see
the Earth in a bright waning gibbous phase. So much light is
reflected from the bigger, blue-white Earth that the lunar night --
that outside of the visible crescent -- is brilliantly lit.
Mercury dominates the evening news, while the giant twin planets
dominate the morning. At the end of the week, little Mercury is
just one day shy of its greatest eastern elongation, when it is 24
degrees to the east of the Sun and nicely visible not far above the
northwestern horizon in twilight shortly after sundown. This is
one of Mercury's best appearances of the year, and a great time to
see it. IN contrast, giant Jupiter and Saturn are seen to the east
in morning twilight. Having passed their Grand Conjunction last
week, closer and speedier Jupiter now begins to pull away to the
east of Saturn. The juxtaposition of the two planets makes the
viewing of planetary motion very obvious. Both, however, move
slowly enough that they will remain in good company with each other
all year as they move gradually toward the evening sky late next
summer.
Unlike the Big Dipper (the main figure of Ursa Major, the Greater
Bear), the Little Dipper gets little respect. Now climbing ever-
higher in the northern sky, only three of its stars are visible
from town, Polaris (at the end of the handle) plus Kochab and
Pherkad, which make the front of the bowl. The others are so faint
that they can be seen only from a dark site. Polaris, the "star of
the sailor," is only 3/4 a degree from the sky's northern rotation
pole, and its elevation above the horizon in degrees is very nearly
equal to the observer's latitude. From polar regions, Polaris is
high in the sky, while from the northern tropics it is low. Below
the equator, it cannot be seen at all. If you keep the star at a
constant elevation while travelling, you track close to a parallel
of latitude, allowing simple navigation across the ocean. Even in
an age of satellite navigation, the stars are always there to
advise. Indeed, satellite navigational systems must still depend
on observations of the sky in order to know the position of the
Earth in its rotation.
