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Time Marches On
At nightfall in March, Leo the Lion jumps into the eastern sky while Aries the flying Ram glides for the western horizon. It's as though these two constellations are proclaiming that March "comes in like a lion and goes out like are lamb."
March's reputation for change is well-deserved. In the Northern Hemisphere, the daylight hours increase by leaps and bounds -- faster than at any other time of the year. And in the Southern Hemisphere, the daylight hours diminish faster in March than in any other month.
At 40 degrees North latitude -- Denver, CO and Philadelphia, PA -- you have about one hour and twenty minutes more daylight at the end the March than at the beginning. And what the Northern Hemisphere gains in daylight, the Southern Hemisphere loses. At 40 degrees South latitude -- New Zealand -- you have about one hour and twenty minutes less daylight by the month's end.
At higher latitudes, the change in daylight hours is even more manifest. At 71 degrees North latitude -- Barrow, Alaska -- you have about four hours and forty minutes more daylight at the end of March than at the beginning. Again, the Southern Hemisphere loses in daylight what the Northern Hemisphere gains, so at 71 degrees South latitude, you have four hours and forty minutes less daylight by the end of the month.
Change of Declination
The northward migration of the Sun in March causes this monumental shift in daylight. In March, the Sun's declination starts out at about 8 degrees South latitude and ends up at about 4 degrees North latitude -- the Sun traveling some 12 degrees north for the month. (Declination refers to the latitude at which the Sun is at zenith -- or straight overhead.)
Let's contrast March to June. The Sun starts out the month at about 22 degrees North latitude and ends the month at a bit over 23 degrees North latitude. The Sun reaches its northernmost point on the June 21 solstice, at 23.45 degrees North latitude. In June, the Sun's declination hardly changes at all, and the same goes for the daylight hours.
Equinoxes and Solstices
The Sun's northward (or southward) daily change in declination is greatest around the equinoxes. About a week before the March 21 equinox, the Sun shines at zenith somewhere around 3 degrees South latitude. On the equinox, the Sun stands at zenith over the Earth's equator (0 degrees latitude). A week after the March equinox, the Sun hovers at about 3 degrees North latitude.
For Denver and Philadelphia, the Sun moving north means a gain of almost 3 minutes of daylight a day, or about 18 minutes a week. For Barrow, it's a gain of about 10 minutes a day, or over an hour a week.
Let's see what things look like three months later at the June (Northern) solstice. A week before and after the June solstice, the Sun's declination is at about 23.25 degrees North latitude. On the June 21 solstice, the Sun's declination is at 23.45 degrees North latitude. At Denver and Philadelphia's latitude, this amounts to about 1 minute more daylight on the day of the June solstice than what you get a week before or after.
The Sun's change of declination is nearly at a stand still at solstice time. That must be agreeable to the folks at Barrow when it's the June (Northern) solstice, with the Sun staying above the horizon for the whole month of June! But Barrow must pay for this extravagance at the other end of the year -- during the Southern solstice -- when the Sun stays beneath the horizon for the whole month of December. BBrrr!!
At the Earth's equator, the Sun's declination has no effect on the length of daylight. There, you pretty much have 12 hours of daylight and 12 hours of night all year long.
The Rite of Spring
But if you live in the temperate or Arctic regions of the Northern Hemisphere, it's time to celebrate the return of the Sun. On the March equinox, the Sun rises due east and sets due west, and everyone everywhere around the world receives about 12 hours of daylight and 12 hours of night. After the March equinox, the Sun rises north of due east and sets north of due west, bringing longer days and shorter nights into the Northern Hemisphere, and shorter days and longer nights into the Southern Hemisphere.
The Sun migrates northward at its fastest clip in March. But this pace is nearly matched in April, whereby the Sun -- and the migrating birds -- continue their flight northward, the Northern Hemisphere basking in the sweet return of spring!
|Another March Adventure: The Solar Pendulum|
copyright 2005 by Bruce McClure
|Sunrise & Sunset Times for the Year|
|Earth View by John Walker|
February 2005 Feature * April 2005 Feature