On Jan. 2, in the heart of winter, we were closest to the Sun than at any point in the year, a fact that begs two questions: Why are we closer? Why aren't we hotter?
If Earth's orbit around the Sun was perfectly circular we would always remain 93 millions miles away, but our path is slightly oval-shaped, called elliptical. So once each year we reach a point farthest from the Sun, and once nearest.
At the farthest point, called aphelion, we are 94.8 million miles away, and at the nearest, called perihelion, 91.1 million miles from the Sun. Perihelion, always in early January, is Jan. 2 this year, and aphelion, in early July, will be July 4.
So if we're now closer to the Sun, why is it cold?
If our orbit was more elliptical, like Mars', then perihelion and aphelion would have a noticeable effect on our temperature, as it does on Mars. But since our distance from the Sun varies by only 4%, it isn't enough to make much of a difference.
Our seasonal temperature variations result from an event that occurred a few billion years ago when the solar system was young. A planet-like object (likely Mars-sized) struck Earth a glancing blow. This cataclysmic event destroyed the other body, and many astronomers theorize that debris blasted from Earth eventually formed our Moon.
The collision also knocked Earth a little wacky, leaving it with a 23-degree tilt on its axis--and this tilt creates our seasons.
During Earth's annual orbit around the Sun, the northern and southern hemispheres take turns tilting toward and away from the Sun. When a hemisphere is tilted toward the Sun, the Sun's warming rays strike that half of Earth more directly and days are longer yielding more daily hours of exposure to the Sun's warmth.
The hemisphere tilted away from the Sun experiences the opposite as Sun's rays strike at a greater angle and there are fewer daily hours of warming sunlight.
Our northern hemisphere is tilted away from the Sun September through March, centering around Dec. 21, the first day of winter. At that same time, the southern hemisphere is tilted toward the Sun so Dec. 21 marks first day of summer below the equator. The other half of the year the seasons are reversed.
So aphelion and perihelion coming within two weeks of the summer and winter solstices is a coincidence that has no bearing on our seasons.
- Jan. 1 Tue. morning: The Moon is to the lower right of the star Spica.
- 4 Fri. morning: The Quadrantid meteor shower peaks in the northeast with little interference from the crescent Moon which rises at 4 a.m. to the right of Venus.
- 5 Sat. morning: The Moon is below the star Antares.
- 7 & 8 Mon. & Tue. mornings: Venus passes nearest Antares.
- 8 Tue.: The Moon is new.
- 9 Wed. morning: Latest sunrise for latitude 30 degrees north.
- 15 Tue.: The Moon is at 1st quarter.
Naked-eye Planets. (The Sun, Moon and planets rise in the east and set in the west due to Earth's west-to-east rotation.) Evening: Mars, well up in the east, dominates the evening sky as Saturn rises around 10 p.m. Morning: Venus is the brilliant "morning star" in the east, Saturn is high in the southwest, and Mars is setting in the west.
Stargazer appears every other week. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco, 76707, (254)753-6920 or paulderrickwaco@aol
.com. See the Stargazer Web site at stargazerpaul.com.