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The Earth is slowing down . . . slightly

Summer arrived during the wee hours Monday at 5 a.m. EDT. At that time, the sun reached its northernmost point in the Northern Hemisphere sky (located in the constellation Gemini), and, after a brief apparent pause, turned around and headed south again.

For most of us with positive latitudes, the sunrise and sunset points are also at their furthest points north, the days are longest, and the nights shortest.

In the Southern Hemisphere, the days are short and the nights long. It is winter there.

In addition to the seasonal variations in the length of the day, two astronomers recently discovered that Earth's rotation is itself slowing, though by a very small amount from year to year.

Astronomer F. Richard Stephenson of Durham University in England, and his collaborator, Leslie V. Morrison of the Royal Greenwich Observatory, discovered a small rotational rate change after digging through years of historical records.

In 1984, they studied historical records of observations made with the aid of telescopes. Most of these records report the precise times of occultations of stars by the moon, a value that is partly dependent on the rotation rate of Earth.

These data revealed that the length of the day has fluctuated by several milliseconds during the previous 350 years.

At the same time, their analysis of records predating the telescope reveal a steady increase in the length of the day at an average rate of 1.7 milliseconds per century over the past 2,700 years.

The most helpful of these records were timings of the different phases of lunar and solar eclipses. Many of these records originate from Babylon and were inscribed on stone tablets between 700 and 50 B.C.

The astronomers also used important accounts of total and near-total eclipses from ancient China and Europe.

The main cause of Earth's slow-down is friction caused by tides, in the ocean and the solid part of our planet.

Non-tidal contributors to the change are thought to include the continuing rise of land that was pushed down by the overlying weight of ice during the last ice age, and changes in sea levels.

Nevertheless, the rotational slowdown is so slight from year to year that it's not noticeable to people at human scales. No need to adjust our sundials yet.

Jeff Kanipe is the editor of Star Date, a bimonthly astronomy magazine published by McDonald Observatory at the University of Texas at Austin.

STAR DATE

PLANETS

Mercury is still low in the west-northwest but sinking fast. It becomes less visible as the month wraps up.

Venus shines brightly through the twilight sky at dawn.

Mars, in Leo, near Regulus, is lower in the west at sunset. It sets around 10 p.m., local time.

Jupiter, in Virgo, is in the southwest at sunset. It sets about four hours after the sun.

Saturn rises late in the evening and is low in the south by dawn.

EVENTS

Tonight: Mars is less than a degree north of the bright star Regulus in Leo this evening.

Thursday: Mars is north of the crescent moon in the evening sky.

Friday: The moon is at perigee (229,507 miles).

Saturday: First quarter moon.

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