Earth and Sky科普英语：September Equinox 秋分
We have an equinox because an imaginary plane extending outward from Earth's equator isn't the same as the plane of Earth's orbit around the sun. Learn more about today's equinox -- on Earth and Sky..
JB: This is Earth and Sky -- Sunday, September 22. No national or cosmic law says the equinox should mark the start of a new season.
DB: And yet we've come to regard solstices and equinoxes as the beginnings of seasons. We pin them down to the minute as Earth moves around the sun. During the night tonight according to U.S. clocks, the sun will cross the celestial equator moving from north to south . . .
JB: And this equinox will be regarded as the beginning of autumn in the northern hemisphere -- and the start of spring in the southern hemisphere. Most people think of the equinox as a day -- but it's really just an instant -- when the sun crosses the celestial equator. That instant comes this evening for you if you're using Central, Mountain or Pacific Time in the U.S. -- or if you're in Hawaii. But it'll be after midnight on your clock tonight if you're in the Eastern time zone of the U.S. or in Europe or Africa.
DB: Signs of the equinox can be seen at any earthly latitude. Due east and due west are horizon markers in the equatorial system -- and since the sun is crossing the plane of Earth's equator tonight, the sun will rise due east and set due west tomorrow. Also, today or tomorrow, watch your shadow. At dawn, it'll point precisely west. At dusk, it'll point precisely east. Happy equinox everyone.
JB: And thanks today to Research Corporation -- a foundation for the advancement of science. We're Block and Byrd for Earth and Sky.
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