Earth at perihelion, Mars and moon, on January 2
January 2, 2010 - ES Tonight
First, Earth’s perihelion – our closest point to the sun – today. It happens every year at this time, despite the fact that January is a winter month for the northern hemisphere. We’re farthest away from the sun in early July, during our northern summer. So you can see that Earth’s distance from the sun isn’t what causes the seasons.
Earth is about 5 million kilometers – or 3 million miles – closer to the sun today than it will be in early July. That’s not a huge change – so, on Earth, it’s mostly the tilt of our world’s axis that creates winter and summer. In winter, your part of Earth is tilted away from the sun – in summer, it’s tilted toward the sun.
Now back to Mars – the world near the moon tonight. Mars also tilts on its axis. And this axial tilt creates the martian seasons, just as on Earth. But Mars orbit is more elliptical than Earth’s, so Mars’ distance from the sun varies more, and that variation does affect martian surface temperatures.
So you have two worlds to think about today. Earth closest to the sun and Mars near the waning gibbous moon from mid-evening until dawn. After rising, the shining couple will glorify the nighttime until morning dawn. The moon and Mars will be in each other’s vicinity on the sky’s dome for these next two nights.
does the tilt at perihelion mean that the southern hemisphere, which is now tilted towards the sun, experiences warmer summers during January than the northern hemisphere does in July?
Oh, and a Happy New Year to everyone!
Sandra,
Great Question! I don’t have a definitive answer for you. If all were equal between the two hemispheres, perhaps the southern hemisphere would have the warmer summers and colder winters. However, the southern hemisphere’s surface is covered over by more ocean and less land mass than in the northern hemisphere. I’m wondering if the greater amount of surface water in the southern hemisphere helps to temper the climate.
Bruce
I read that Mercury’s eliptical orbit around the Sun goes through precession over time. Does the Earth’s eliptical orbit do this too and if so, why and what time interval are we looking at? Would the Earth’s perihelion date change in the future?
john the earth does go through precession and i believe it is a 20,000 year cycle, dont hold me to that number but i think that is it. and that could change the earths perihelion i would assume.
John and marc,
Both of you seem to have the right idea. According to the US Naval Observatory article at http://aa.usno.navy.mil/faq/docs/seasons_orbit.php, the perihelion cycle is some 21,000 years in duration. Some 5,000 years from now, perihelion will happen around the March equinox. Some 10,000 years from now, perihelion will occur near the June solstice. In 15,000 years, perihelion will happen in close vicinity with the September equinox. And in 20,000 years, perihelion will take place around the time of the December solstice.
Bruce