Each planet revolves
around the Sun in an
elliptical path, with the Sun occupying one
of the foci of the ellipse.
-- Kepler's 1st Law of Planetary Motion
June 30, 2000 -- On the 4th of July, Earth will be
at its greatest distance from the Sun in the year 2000. But don't
expect any sudden relief from the heat. Northern summer will
be as warm as ever despite our arrival at a distant part of Earth's
orbit that astronomers call "aphelion."
"Earth moves in an elliptical orbit," says George Lebo, an astronomer at the NASA Marshall Space Flight Center. "We make our closest approach to the Sun (147.5 million km) in January -- that's called perihelion. We're at the greatest distance (152.6 million km), or aphelion, in July. This year aphelion falls on Independence Day holiday in the U.S."
Above: After sunset on the 4th of July, a slender crescent moon seen low in the western sky will serve as a delicate celestial backdrop for Independence Day fireworks in the U.S.A. Although the Earth will be at its greatest distance from the Sun on July 4th, northern summer will be as warm as ever. Appropriate attire for watching fireworks still includes shorts and a t-shirt. Image credit: Duane Hilton.
"Sunlight falling on Earth is about 7% less intense in
July than it is at our closest approach to the Sun in January,"
says Roy Spencer of the Global Hydrology and Climate Center.
You might expect northern summer to be cooler because it occurs
when Earth is farther from the Sun. Not so, explains Spencer.
"The oceans and land on Earth are not evenly distributed
around the globe. The northern hemisphere has more land; the
southern hemisphere has more water. This tends to moderate the
impact of differences in sunlight between perihelion and aphelion."
On planets like Mars with orbits much more eccentric than ours, the varying distance to the Sun can have a big influence on seasonal conditions. For example, northern summer on Mars lasts 24 days longer than winter because the planet is close to aphelion when the north pole is tilted toward the Sun. Planets move more slowly at aphelion than they do at perihelion (see Kepler's 2nd Law of planetary motion) and, so, seasons occurring near aphelion last longer. During the long martian summer, so much CO2 frost at the north pole sublimes into gaseous form that the global pressure of Mars' atmosphere increases by up to 30%.
Imagine leaving for a lunchtime picnic on the 4th of July
only to arrive at the park and discover the humid summer air
to be one-third denser than normal! This won't happen on Earth
because our orbit is more nearly circular than the Red Planet's.
Earth has one of the most circular orbits in the solar system. Only Neptune and Venus follow more perfectly circular paths around the Sun. The other six planets trace paths that are significantly elliptical. Pluto, the planet with the greatest orbital eccentricity, follows a path so lopsided that it sometimes travels closer to the Sun than Neptune. Just last year Pluto ended a 20-year stint as the eighth planet when it crossed Neptune's orbit in February on its way back to the outer solar system.
After Pluto, the second and third most elliptical orbits belong to Mercury and Mars, respectively. As this diagram illustrates, their paths around the Sun (solid lines) depart significantly from that of a circle (dotted lines). Future astronauts visiting Mars or Mercury would notice that the apparent size of the Sun varies a great deal throughout the year. On Mars, for example, the aphelion Sun is 0.30 degrees across. At perihelion it would grow to 0.36 degrees in diameter, an increase of 20%.
Right: The orbits of Mercury (red), Earth (blue) and Mars (black). The solid lines indicate each planet's elliptical path around the Sun. The dotted lines show circular paths with the same mean separation from the center. Earth is almost exactly the same distance from the Sun at aphelion and perihelion, but the orbits of Mars and Mercury depart significantly from a circle. For more information, please visit Bridgewater College's Interactive Planetary Orbits web site.
The high eccentricity of Mercury's orbit, coupled with its peculiar rotation (it rotates on its axis 3 times for every 2 circuits around the Sun), would produce very strange effects for an observer there. At some longitudes the Sun would rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon, decreasing in size as it set. All the while the stars would be moving three times faster across the sky. At the instant of perihelion, the Mercurian Sun would appear to be 1.6 degrees wide. At aphelion, the solar disk would shrink to 1.1 degrees, about twice as large as the half-degree solar diameter that we see from Earth.
Back on Earth, if you plan to spend the 4th of July holiday outdoors, don't bother staring at the Sun to see if it looks any smaller. The blindingly bright solar disk will subtend an angle that is 1.7% less than average but you won't be able to discern the difference with the naked eye. Instead, keep an eye on the western sky just after sunset. A delicate waxing crescent Moon will grace the horizon and possibly serve as the spectacular backdrop for Independence Day fireworks.
closest point to the Sun
Aphelion Distance |
farthest point from the Sun
|Notes: 1 AU, the average distance from the Earth to the Sun, equals 93 million miles or 150 million kilometers. The eccentricity of a planet's orbit measures how much it departs from a perfect circle. Orbits with zero eccentricity (e = 0) are circular; orbits with eccentricities close to 1 (e ~ 1) are long and skinny. Planetary orbits tend to be almost circular while comets and many asteroids follow more eccentric paths.|
The Global Hydrology and Climate Center is a joint venture between government and academia to study the global water cycle and its effect on Earth's climate. Jointly funded by NASA and its academic partners, and jointly operated by NASA's Marshall Space Flight Center in Huntsville, Ala., and the University of Alabama in Huntsville, the Center conducts research in a number of critical areas.Web LinksEarth's Seasons - a table of aphelia, perihelia, solstices and equinoxes from the US Naval Observatory
Daily Earth Temperatures from Satellites -View global atmospheric temperature trends at different layers of the atmosphere, courtesy of the Global Hydrology and Climate Center.