Happy Fall - if you're living in the Northern Hemisphere. Otherwise - Happy Spring for my Southern Hemisphere friends.
What do we wish to our fellow humans orbiting the Earth in the International Space Station?
Anyway, today marks the September Equinox, the start of Fall in New England, at least. The word "Equinox" means "equal night." That is, theoretically, we'll have a day with 12 hours of sunshine and 12 hours of night. Equal.
In practice, things aren't so equal. Because the Earth has an atmosphere (I'm not complaining), the Sun will (by refraction) actually continue to appear above the horizon even when it has dipped just below. And the Sun's light gives us dawn and dusk even at those below the horizon times. Not exactly equal.
Why is the Equinox happening today at 10:21 A.M., then?
The Earth spins on its axis, but that spin axis is tilted by about 23½º. During the (Northern Hemisphere's) Summer Solstice, the North Pole is angled most towards the Sun, which is overhead at a latitude of 23½ºN. About three months after the Summer Solstice the Sun crosses the "Celestial Equator," which is the Earth's Equator projected into space. At this moment we turn to Fall.
Because of leap years, the Equinox is not always on September 22.
All this orbital dynamics is often more easily explained using a planetarium. If you're interested in learning more about Blue Star Planetarium, check out the website here, or email your questions any time.
Marc
What do we wish to our fellow humans orbiting the Earth in the International Space Station?
Anyway, today marks the September Equinox, the start of Fall in New England, at least. The word "Equinox" means "equal night." That is, theoretically, we'll have a day with 12 hours of sunshine and 12 hours of night. Equal.
In practice, things aren't so equal. Because the Earth has an atmosphere (I'm not complaining), the Sun will (by refraction) actually continue to appear above the horizon even when it has dipped just below. And the Sun's light gives us dawn and dusk even at those below the horizon times. Not exactly equal.
Why is the Equinox happening today at 10:21 A.M., then?
The Earth spins on its axis, but that spin axis is tilted by about 23½º. During the (Northern Hemisphere's) Summer Solstice, the North Pole is angled most towards the Sun, which is overhead at a latitude of 23½ºN. About three months after the Summer Solstice the Sun crosses the "Celestial Equator," which is the Earth's Equator projected into space. At this moment we turn to Fall.
Because of leap years, the Equinox is not always on September 22.
All this orbital dynamics is often more easily explained using a planetarium. If you're interested in learning more about Blue Star Planetarium, check out the website here, or email your questions any time.
Marc
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