|
Indian summers around the world, slowly spinning moons
Q:
During September in Cyprus (an island in the eastern Mediterranean about 40
miles [64 km] south of Turkey), the temperature suddenly rises at least 3° to
4°C (5° to 7°F) for a week or two. Does the Earth getting closer to the Sun
cause this "Indian summer?" Are there any strange air movements north and south?
(Viken, Nicosia, Cyprus)
 Indian summer in Maine [Corel]
A: No. The Earth getting close to the Sun doesn’t cause Indian
summers. Instead, stalled high-pressure areas fuel the formation. But, yes.
Winds do move from south to north in the Northern Hemisphere (and north to south
in the Southern). The same phenomenon creates Indian summers in the US and
elsewhere.
Take New England, for example. During the fall, high-pressure
areas sweep out of the Northwest, move across the country, and sometimes stall,
off the Atlantic Coast. This creates a high pressure over New England.
Higher pressure pushes air towards lower pressure, causing
winds. Because of the Earth’s spin about her axis, the winds rotate counterclockwise
about high-pressure zones in the Northern Hemisphere and, hence, about the New
England high. The gyrating winds curve south, and fetch warm air north to New
England. This creates an Indian summer there that lasts until the next strong
low pressure and its cold front juts across New England. Sometimes many Indian
summers occur in the fall and sometimes none at all.
"My take on all this is that Indian summers are part of the
regular parade of weather systems that occur throughout the year," says
David Schultz,
meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma. "If
you get unusually warm weather late in the fall, people want to give it a name,
but it’s really just part of the normal climatology."
Wondering about other parts of the world, I asked
Bob
McDavitt, a New Zealand meteorologist, if Indian summers occur in the
Southern Hemisphere. "Yes," he said. High-pressure bands (called subtropical
ridges) circle the globe normally near 30° N and 30° S. In the
fall, however, they follow the Sun to somewhat higher latitudes — 35° to
45° — and cause the Indian summer phenomenon. So, it’s a warm delight
that occurs worldwide. "We in the Southern Hemisphere get it in April or May,
says McDavitt"
By the way, the name "Indian summer" has been around at least
since 1778. According to the Glossary of Meteorology, the term probably refers
to Indians using the warm days to increase winter stores.
Further Reading:
USA Today: High
pressure system, graphic
National Weather Service, NOAA: Just what is Indian summer and did Indians
really have anything to do with it by William R. Deedler, 1996
American
Meteorology Society: The Glossary of Meteorology, 2000.
The Weather Doctor: The halcyon days of Indian summer by Keith Heidorn
Q: What is the relationship
between the day and the year of a body that has captured rotation? Also, what
other bodies of the solar system have captured rotation? (Sally, Australia)
A: A body (for example, the Moon) that has captured rotation
is a natural satellite that orbits a primary body (for example, Earth). Such a
body rotates once about its axis in the same length of time that it orbits the
primary. The same hemisphere always faces the primary body.
If you define a "day" as the time the body takes to spin once
about its axis and a "year", the time the body takes to orbit the primary, then
the body’s day equals its year.
Our
Moon is such a body. It takes the Moon 27.32 days to rotate about its axis and
also 27.32 days to orbit Earth. Consequently, the "nearside" of the Moon always
faces Earth.
 Our Moon’s nearside always faces Earth: the farside is to
the left of the concentric circular Orientale basin, near the picture center.
The nearside is to the right. [NASA]
To answer your second question: Almost all the 138 moons of
our solar system seem to have captured rotation. Their "days" equal their
"years."
Here’s a partial list:
- Earth (the Moon)
- Mars (Phobos, Deimos)
- Jupiter (Amalthea, Thebe, Io, Europa, Ganymede, Callisto)
- Saturn (Epimetheus, Janus, Mimas, Enceladus, Tethys, Dione,
Rhea, Titan, Iapetus)
- Uranus (Ariel, Umbriel, Titania, Oberon)
- Neptune (Triton)
- Pluto (Charon)
This
list doesn’t include many moons because we lack rotational data. We know how
long it takes them to orbit their planet (their "year") but not their "day".
Saturn’s 16th moon out, Hyperion, however, is an
exception. Her axis of rotation wobbles too chaotically for Saturn to capture
that moon’s rotation.
 Phoebe, Saturn’s icy dark moon, as seen from Cassini’s
flyby, 2004. [NASA/JPL/Space Science Institute]
Also, Saturn’s last moon out, dark Phoebe, is another oddball.
She does not exhibit captured rotation either. She may be a captured comet.
Further Reading:
WonderQuest:
Moon spin — what causes captured rotation (hint: the tides)?
Royal Observatory Greenwich: The Moon
Views of the
solar system by Calvin J. Hamilton: Table listing the orbital and rotational
periods of the moons in the solar system
The nine
planets by Bill Arnett: Hyperion
The nine
planets by Bill Arnett: Phoebe
The
universal book of astronomy by David Darling: Gravitational lock
(Answered Sep. 17, 2004)
|
