|Home Page||Astronomy Links||Stars Page||Astronomy Articles|
Annually, on or near June 21, the Earth's northern axis points most toward the Sun for the year. And the Earth's southern axis points most away. It's the summer solstice in the Northern Hemisphere, giving us our longest day and shortest night of the year. And it's the winter solstice in the Southern Hemisphere, giving that part of the world its shortest day and longest night.
Diagram of seasons on right courtesy of Wikimedia Commons. You can see that the Earth's northern axis points most toward the Sun on June 21, and most away from the Sun on December 21. On the equinoxes, neither the Earth's North Pole nor South Pole leans toward the Sun or away from the Sun.
In short, the Earth's greatest axial tilt relative to the Sun happens on the solstices, whereas the Earth's axis doesn't tilt at all toward or away from the Sun on the equinoxes.
Another Dynamic: Tilt of Axis Relative to Earth's Orbit
We can also talk about the tilt of the Earth's axis relative to Earth's orbital path around the Sun. On the solstices, the Earth's northern axis doesn't point in the direction of Earth's orbital motion or away from it. The same holds for the Earth's southern axis on the solstices: it neither leans toward nor away from Earth's direction of motion.
To recap, the Earth's axis doesn't tilt at all relative to the Earth's orbital motion on the solstices. Yet, on the September equinox, the Earth's northern axis most tilts in the direction of Earth's orbital motion, whereas the southern axis points most directly behind. The shoe's on the other foot on the March equinox: the northern axis points most behind and the southern axis points most ahead.
This is why we tend to see the most sporadic meteors - meteors not associated with any particular meteor shower - around the time of the autumn equinox. That's when the northern (or southern) axis rides on the forward-moving side of our planet as we circle the Sun.
Visualizing Earth's "Windshield" - Orbital Path through Space
I suspect that it's difficult for most people to envision the direction in which we're traveling around the Sun. Let me try to make it intelligible. This point - the Earth's "windshield" - always lodges 90 degrees west of the Sun. As bugs are more likely to splatter on the windshield than the rear window of a car, sporadic meteors are more likely to blaze across "windshield" rather than the "rear window" of Earth.
At solar noon (or midday) on the June solstice, the "windshield" faces due west on the horizon. Six hours after midday - at around 6 p.m. local clock time - the Earthly "windshield" faces due north (though beneath the horizon). At midnight, the "windshield" faces due east on the horizon. Six hours after midnight - around 6 a.m. local clock time - the "windshield" faces due south.
Highest Up Daily at 6 Hours Before Noon
It's around 6 a.m. local clock time that the windshield is looking highest upward in the sky for the day. The higher up that we are looking through the windshield, the more sporadic meteors that we are likely to see - given dark skies.
Then 6 hours later, at noon, the Sun shines due south at its highest point for the day. As long as you live north of the tropic of Cancer, the Sun is always due south at noon, and the point marking the Earth's orbital direction in space is always due south at 6 hours before noon.
Highest Up 6 Hours Before Noon on Autumn Equinox
Most of us know that the Sun climbs highest up in the sky for the year at noon on the summer solstice. In like vein, the point in the sky that marks our orbital direction through space climbs highest up in the sky for the year at 6 hours before midday on the autumn equinox.
Cimbs Above Horizon Before Midnight on Autumn Equinox
Also, the Sun rises a maximum time before true solar noon on the day of the summer solstice. Similarly, the point toward which we are orbiting rises a maximum time before the 6 o'clock hour on the autumn equinox. Whereas this point rises at midnight on the solstices, it rises a maximum time before midnight on the autumn equinox and a maximum time after midnight on the spring equinox.
This "radiant point" for sporadic meteors soars highest in the sky during the dark hours before dawn on the autumn equinox, so that's why the greatest numbers of sporadic meteors tend to fly before dawn in late summer and autumn.
At the June solstice, the Earth's axis doesn't lean forward or behind, relative to the Earth's orbital motion. After the June solstice, however, the Earth's northern axis starts to lean in the direction of Earth's orbital motion, not to start trailing the Earth's motion till after the December solstice.
copyright 2010 by Bruce McClure
|Question #4 on sporadic meteors answered by the AMS|
May 2010 Feature * July 2010 Feature