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September 2004 Feature: Southbound Express



Oftentimes, we're told that the Sun rises in the east and sets in the west. Strictly speaking, the Sun only rises and sets due east and west on the equinoxes. After the March equinox and before the September equinox, the Sun rises and sets a bit north of due east and west. After the September equinox, the Sun rises and sets somewhat south of east and west.

On the June solstice, the Sun reaches its northernmost point for the year, residing at about 23.5 degrees north of the equator. For both the northern hemisphere and the southern hemisphere, the Sun rises and sets at its northernmost points for the year. At this juncture, the northern hemisphere has its longest day and shortest night for the year, whereas the southern hemisphere must endure its shortest day and longest night.

As the seasons roller coaster back and forth, bringing the Sun to its northernmost point on the June solstice and its southernmost point on the December solstice, the pace at which the Sun changes declination (north or south direction) varies dramatically throughout the year. Near the solstices, it's as if the Sun is riding in a roller coaster that's struggling to reach the crest of a hill, slowing down to a near stand still. The Sun's daily change of declination is nearly imperceptible for about two weeks -- the week before and after the solstice.

On the other hand, as we bypass a solstice and approach an equinox, it's as though the roller coaster picks up speed as it goes downhill. The Sun's daily change of declination is always swiftest and most obvious at equinox time.

In the second half of September -- during the two weeks centered around the September 22 equinox -- you can watch the Sun's southward journey along the horizon. If you make a mental note of where the Sun rises and sets, chances are you'll observe the Sun's change of position along the horizon after a few days.

At a middle latitude of 45 degrees, either north or south of the equator, the Sun rises and sets about one half degree south of where it did the day before. (For reference, the diameter of the Sun equals about one half degree.) In our northern hemisphere, when the Sun rises farther south that it did the day before, that means a later sunrise, and when the sun sets farther south than it did the day before, that means an earlier sunset. All things added up, we lose about three minutes of sunlight daily, or about twenty-one minutes weekly at 45 degrees north latitude; people residing at 45 degrees in the southern hemisphere gain in daylight what we lose -- about three minutes a day, or twenty-one minutes a week.

All over the world, the Sun travels at its fastest pace southward for the year during the last two weeks in September. Yet, at latitudes closer to the equator than 45 degrees latitude, the Sun's sunrise and sunset points change by less than one half degree daily. The loss and gain of daylight become less severe as you approach the equator, where the daylight hours remain unchanged throughout the year.

At latitudes closer to the Poles than 45 degrees latitude, the Sun's sunrise and sunset points change by more than one half degree daily. For example, at Barrow, Alaska, which resides at 71 degrees north latitude, the sun rises and sets a whopping 1.2 degrees south of where it did the day before, with the daylight hours diminishing by about nine minutes daily, or some 64 minutes weekly. Meanwhile, at the same latitude in the southern hemisphere, the Sun also rises and sets 1.2 degrees south of where it did the day before -- except that south of the equator, it's a nine-minute gain of sunlight daily, or some 64 minutes a week.

Although the Sun travels southward most rapidly during the last two weeks of September, the Southbound Sun speeds on almost as quickly all through September and October, a telltale sign of the change of seasons.


copyright 2004 by Bruce McClure


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