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By Alan MacRobert
Adapted from Sky & Telescope

ONE OF THE FASCINATIONS of skywatching is that it's different wherever you go. And certainly the biggest change occurs when you go from the Northern to the Southern Hemisphere.

Travel to the midsouthern latitudes of Australia, and the Big and Little Dippers disappear while Alpha Centauri, the Southern Cross, and other south circumpolar constellations become permanent features of the night. The Sun and Moon cross the sky from right to left instead of left to right, and summer and winter are reversed.

Travel to high latitudes in either hemisphere, and different effects show up. Twilights lengthen, and night never gets truly dark at all for awhile around midsummer.

Many interesting latitude effects show up in the three different versions of our annual Skygazer's Almanac.

Every January since 1942, Sky & Telescope has published a version of the Skygazer's Almanac drawn for latitude 40° north. Now January issues destined for the United Kingdom and Europe contain a separate edition for 50° north, and those headed south of the equator feature a Southern Hemisphere edition drawn for latitude 30° south. The 1998 editions of these newer versions are shown in miniature below. (Click on each image for a larger view.)

SGA, 50 deg N SGA, 30 deg S

Dates of the year run vertically down the side, and time of night runs horizontally. Each curved or slanting line tells the time of night when an event happens throughout the year. The left edge represents the time of sunset, and the right edge marks sunrise. The times are in Local Mean Time, which probably differs from your civil (clock) time.

The most obvious difference between the northern and southern charts is the reversal of summer and winter between the two hemispheres! June and July (midway down the chart) have short summer nights in the Northern Hemisphere and long winter nights in the Southern Hemisphere.

Another difference is the length of twilight. This has to do not with the change in hemisphere but the difference between being 30° or 50° from the equator. Night falls fast at low latitudes. Farther from the equator, twilight is always more prolonged.

This effect is greatest around the summer solstice. Skywatchers near 50° latitude -- mid-Canada, southern England, northern France, and central Germany, for instance -- get no true darkness at all for six weeks! Even at midnight the Sun skims below the northern horizon by less than 18°, the definition of astronomical twilight.

Farther north at Arctic latitudes, the Sun itself never sets around these dates. The territory where this occurs is bounded by the Arctic Circle. So perhaps we could define a "Subarctic Circle" at 48.6° latitude, above which there are at least a few nights of perpetual twilight every summer.

Other effects show up on closer looking. Mercury stays buried in the glow of dusk and dawn at high northern latitudes. There's a legend (probably false) that Copernicus himself, living at 54° north near what is now Gdansk, Poland, never saw Mercury at all. Skywatchers in Australia and South Africa, on the other hand, find Mercury standing well above the horizon in total darkness several times a year.

Part of this has to do with the shorter twilights near the equator regardless of the hemisphere. But another effect is at work too. Mercury has a rather eccentric (elliptical) orbit, so at some elongations it appears farther from the Sun than at others. But as luck would have it, the farthest elongations come at times of year when the ecliptic lies at a low angle to the sunrise or sunset horizon for midnorthern observers. So the benefit of Mercury's largest elongation is canceled out. In the Southern Hemisphere, however, the two effects add together. So Mercury is high and obvious during its cold-weather apparitions.

Other effects can be found on the charts too. Venus has a high apparition in the Southern Hemisphere when it has a low one for northerners. In 1998 skywatchers near 50° north saw Jupiter rise during twilight throughout its entire 1998 apparition, no matter what time of night it came up. What else can you discover by scrutinizing the Skygazer's Almanac?

Alan MacRobert is an associate editor of Sky & Telescope magazine and an avid backyard astronomer.

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