Thursday is the first day of fall.
It means cooler nights and shorter days, but what is the science behind the changes. Tennessee Tech Physics Department Chair Steven Robinson said it’s due to the sun dipping below the celestial equator.
“The Earth is getting into the southern part of its orbit around the sun, which means that the sun is spending less time in the sky for us,” Robinson said. “There is less heating effects on the earth.”
Robinson said the celestial equator is just a projection of earth’s equator into space. Robinson said the moment the sun passes that point is called the Autumn Equinox. Robinson said on this day, the sun will spend half of the day below the horizon and the other half above the horizon.
“It is not quite 12 hours of darkness and 12 hours of sunlight, because we have dusk and dawn when the sun is actually below the horizon but we still have light,” Robinson said.
Robinson said the equinox is also not a full day. Robinson said it happens at a precise time when the sun crosses the equator. That will happen at 8:03 p.m. Thursday.
Robinson said the day of the fall equinox moves on a calendar due to leap years. However, it always happens at the same time universally, Robinson said.