|
 July
14, 2000 -- The longest-lasting lunar eclipse in 140 years
takes place this Sunday morning when the Moon plunges into the
deepest part of Earth's shadow for nearly two hours. Judging
from the appearance of the Moon during the
last lunar eclipse in January 2000, astronomers expect the
Moon to turn a striking reddish-orange color. The best sites
for viewing this event will be in Hawaii, the Pacific Ocean (including
Oceania) and the Far East. Early risers near the Pacific coast
of the US will also be able to catch a glimpse of the amber-shaded
Moon as it sets over the southwestern horizon, but the eclipse
doesn't begin until well after moonset for most North Americans.
[eclipse visibility
charts and tables]
Above: Artist Duane
Hilton's rendition of an eclipsed Moon sinking below the
horizon of a distant Pacific isle.
"This eclipse is especially long because the Moon passes
right through the middle of the darkest part of Earth's shadow.
In places like Australia, where the entire eclipse can be seen,
totality will last for 108 minutes," says George Lebo, an
astronomer at the NASA Marshall Space Flight Center.
 Unlike
total solar eclipses, which persist for a few minutes at most
and can be seen only along a very narrow track measuring a few
kilometers across, lunar eclipses are slow and they are visible
over much of our planet when they happen.
A lunar eclipse takes place when the full Moon passes through
our planet's shadow. The Earth's shadow has a conical shape with
two parts. The umbra (on the inside) is very dark while
the penumbra (on the outside) is very weak. Lunar eclipses
are considered total when the Moon passes completely into the
umbral shadow.
"You might think that the Moon would be completely
dark at totality, but it's not," continued Lebo. "Our
planet's atmosphere refracts sunlight into the umbral shadow,
so even at maximum eclipse the Moon is weakly illuminated."
The exact appearance of the eclipsed Moon depends on how much
dust and clouds are present in Earth's atmosphere. Total eclipses
tend to be very dark after major volcanic eruptions since these
events dump large amounts of volcanic ash into Earth's atmosphere.
During the total lunar eclipse of December 1992, dust from the
eruption of Mount Pinatubo in the Philippines rendered the Moon
nearly invisible. Since no major volcanic eruptions have taken
place recently, the Moon will probably take on a bright coppery
color during the long total phase.
CONTINUES
AFTER SIDEBAR
Lunar
Eclipses on the Moon Future lunar colonists will undoubtedly
mark their calendars with dates when the Moon is expected to
glide through Earth's shadow. Earthlings would consider those
events to be lunar eclipses, but from the point of view of a
Moon-dweller they will be solar eclipses instead. A colonist
standing on the Moon inside Earth's penumbral shadow would see
the disk of the Sun partially covered by our planet. From a viewing
site inside the umbral shadow, they would see a total eclipse
of the Sun.
 What would it look like? From a vantage
point on the Moon, the Earth appears to be 1.8 degrees wide while
the Sun subtends an angle of 0.5 degrees. A total solar eclipse
would therefore look much like images from the Solar
and Heliospheric Observatory's C3
coronagraph, which uses a 1.85 degree occulting disk to reveal
the Sun's faint outer corona.
Solar eclipses on Earth are notable
because the Moon and the Sun appear to be almost exactly the
same size (0.5 degrees wide). This leads to phenomena like Baily's
Beads and the Diamond Ring effect that would not be seen by eclipse
watchers on the Moon. However, any total solar eclipse would
be widely visible on the Moon. On Earth they are restricted to
small parts of our planet's surface. |
To see this weekend's eclipse from the US west coast, go outside
at 3:30 to 4:00 a.m. on Sunday morning and look southwest. The
Moon enters the Earth's outer (penumbral) shadow at 3:47 a.m.
PDT. By 5 o'clock in the morning the dark umbral shadow will
begin to take a distinctive bite out of the bright lunar disk.
At that time, the setting Moon will be at an altitude of just
10 degrees, so a clear view of the western horizon is essential.
 Left: During this weekend's eclipse the full
Moon will be approximately 20 degrees from the
star clouds of the Galactic Center in Sagittarius and just
two Moon widths from the 5.4th magnitude asteroid Vesta. The
asteroid and summer Milky Way may be visible during totality.
This star chart shows the appearance of the southwestern sky
at 3:30 a.m. Sunday morning, July 16, from San Francisco, CA.
Skywatchers in places like Hawaii and Australia where the entire
total phase of the eclipse can be seen are in for a double-extra
treat if they watch the event from a dark sky site. First, the
totally eclipsed Moon will lie approximately twenty degrees east
of the Sagittarius
star clouds. During partial phases of the eclipse these clouds
will be invisible, but during totality the beautiful summer Milky
Way should be easy to see. Also, the asteroid 4
Vesta will appear approximately one degree from the full
Moon. Coincidentally, Vesta is making a rare appearance as a
naked-eye object on July 16th as it nears solar opposition. (An
ephemeris of Vesta is available from the Harvard Center for
Astrophysics.) Sharp-eyed observers in dark rural areas may be
able to spot the minor planet during the total phase of the eclipse.
To learn more about the geographic visibility of the eclipse
and when to look, please consult the charts and tables below.
The table is based on a similar one prepared by Sky
& Telescope magazine. The diagrams are from Fred Espenak's
comprehensive Eclipse
Home Page at the NASA/Goddard Space Flight Center.
Total Lunar Eclipse of July 16, 2000
|
Event |
PDT |
Alaska |
Hawaii |
New Zealand |
East Australia |
|
Moon enters penumbra |
|
|
|
|
|
|
First shading visible |
|
|
|
|
|
|
Partial eclipse begins |
|
|
|
|
|
|
Total eclipse begins |
|
|
|
|
|
|
Mideclipse |
|
|
|
|
|
|
Total eclipse ends |
|
|
|
|
|
|
Partial eclipse ends |
|
|
|
|
|
|
Last shading visible |
|
|
|
|
|
|
Moon leaves penumbra |
|
|
|
|
|
|
Notes: This
table is based on a similar one published by Sky & Telescope
magazine. All times in the table are local times on July 16 except
for times marked by an asterisk, which refer to July 17, 2000. |
 |
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