Astro Bob blog: A little eccentricity makes life more interestingEarth is farthest from the sun this week for the year, but you'd never know it. Watch a very nice crescent moon - Pleiades pairing tomorrow morning.
By: Bob King, Duluth News Tribune
A little eccentricity makes life more interesting
Early tomorrow morning, catch a nice crescent moon alongside the Seven Sisters star cluster. This map shows the sky as you look northeast at the start of dawn. Created with Stellarium
Miss winter yet? The nonstop heat in our region almost makes me wish for those chilly blasts from the north. While that season is far, far off, you can get a free sample of what's to come by getting up tomorrow morning around dawn and looking east. You'll be greeted by a thin crescent moon very near the Seven Sisters or Pleiades star cluster. What a fine sight they'll be in binoculars -- a sliver of moonlight, eerie earthshine and the brilliant stars of winter's pre-eminent star cluster a-sparkle in a compact arrangement.
The Pleiades cluster returns each year around the time of the summer solstice, reborn in dawn’s pale blue light. It's refreshing to see this "Little Dipper" of stars in the middle of a heat spell.
Yesterday Earth was furthest from the sun for the year, a point in our orbit astronomers call aphelion (ap-HEE-lee-un). Not that you'd notice a difference in temperature. I sure didn't. We're only about 3% closer to the solar system's furnace than usual. Not far enough, I'm afraid, to cool things down. As we learned in this earlier blog, the reason we have seasons has everything to do with the tilt of Earth’s axis, not with our changing distance from the sun. That’s not necessarily true with all planets as we’ll soon see.
The Earth's orbit is elliptical with the sun slightly off-center. This causes the Earth's distance and orbital speed to vary during a year's time. We're furthest from the sun this week (aphelion) and moving more slowly than in January when Earth is at perihelion or closest to the sun. The orbit diagram is exaggerated for effect. Illustration: Bob King
Every planet, asteroid and most comets orbit around the sun in ellipses which resemble ovals. Some ellipses are long and thin like cigars, while others are nearly circular. How “squashed” an ellipse is is called its eccentricity. An ellipse with an eccentricity close to one, say 0.8 or 0.9, is stretched out like rubber band. Ellipses with eccentricities closer to "0" are nearly circular. If Earth’s orbit were a circle there’d be no aphelion or perihelion, since our distance from the sun would never change. Planetary orbits all have the sun at one focus of the ellipse which is offset from the center depending upon how eccentric the ellipse is. Really stretched out ellipses have the sun well off to one side; nearly circular ones have the sun almost exactly at center.
Examples of ellipses with different eccentricities
According to Newton’s Law of Gravity, a planet moves at its maximum speed when closest to the sun and at its minimum when farthest. When Earth's at aphelion in summer we move more slowly than at perihelion in winter. You can imagine how this affects the length of the seasons. Summer is the longest with 94 days. Spring has 93, and fall and winter each 89. Maybe there's a deeper Newtonian truth to those hazy, lazy days of summer.
Earth's orbit, while elliptical, is so close to a circle that our planet’s distance varies only slightly during the year. All orbits are not created equal however. Below is a list of the planets' orbits and their eccentricities from least to most eccentric:
1. Venus (.0068) -- the most nearly circular orbit of all eight planets
2. Neptune (.0086)
3. Earth (.0167)
4. Uranus (.0472)
5. Jupiter (.0485)
6. Saturn (.0556)
7. Mars (.0934)
8. Mercury (.206)
An ellipse has two "foci" on either side of center that are used to define its shape. In the solar system the sun is one of those foci while the other is unoccupied.
Notice that Mars (right) is almost last on the list. Like Earth, Mars has seasons, but its orbit is much more squished (eccentric) with a considerable difference between perihelion and aphelion distances. At closest, Mars is 127 million miles from the sun and 153 million at farthest -- a difference of 21%. Combined with its change in speed around its orbit, Mars seasons are strongly affected.
When it’s winter in Mars northern hemisphere, the planet is near perihelion (closest to the sun), so the extra heat it receives makes for a comparatively short, mild winter. Summer, which occurs around aphelion, is relatively long and cool. For the southern hemisphere, it’s just the opposite. Summers are short and hot with long and cold winters.
If we ever populate Mars, our descendents will be able pick their weather pleasure by choosing to live in one hemisphere or the other.
(Mars image credit: NASA/ESA)