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.
 

A common misconception is that Earth's varying distance from the Sun causes the four seasons. In fact, the 23.5-degree tilt of our planet's spin axis is more important. The tilt of the north pole toward the Sun in June causes summer north of the equator, while summer south of the equator comes six months later when the south pole is facing the Sun. The ellipticity of Earth's orbit does cause a small change in solar heating from July (aphelion) to January (perihelion), but it's not the dominant factor in shaping seasonal weather patterns.

"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."
 

Editor's Note: Do you have trouble remembering the difference between perihelion and aphelion? An old astronomer's trick is to recall that the words "away" and "aphelion" both begin with the letter "A".

At aphelion the land-crowded northern half of our planet is tilted toward the Sun. For a given amount of sunlight, land warms up more than water (in other words, land has a lower heat capacity). Sunlight is therefore more effective at raising the temperature of the northern hemisphere. This results in the surprising fact that northern summer is a little warmer than southern summer even though Earth is farther from the Sun in July.

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.
 

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The annual size of the Sun changes most on Pluto. An observer there might not notice the difference, however, because the Sun always seems so small from a vantage point in the outer solar system. The maximum size of the solar disk from Pluto is a mere 1 arcminute across, or 30 times smaller than the Sun we see from Earth. The human eye can't distinguish objects separated by angles less than about 50 arcseconds (0.8 arcminutes), so even at perihelion, the Sun would appear as a starlike point to a human observer on that icy world.

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.

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.