In 1609, Johannes Kepler discovered that planets (including Earth) have elliptical orbits. This means our average 93 million-mile distance from the sun (all figures herein are approximate) is constantly increasing or decreasing throughout the orbital year except at the moments when Earth is either closest at 91.5 million miles to the sun, or farthest at 94.5 million miles from the sun. Our farthest distance (known as aphelion) occurs this year on July 6, and the closest distance (known as perihelion) took place on Jan. 2.

But how can Earth be three million miles closer to the sun at perihelion in the coldest season, winter, than we are at aphelion in the warmest season, summer? (The seasons referred to herein apply only to the Northern Hemisphere to avoid confusion.)

First, summer is the longest season, lasting 94 days. This gives summer five extra days of sunshine (six percent more) than the shortest winter season's 89 days. Next, Earth's axial tilt allows longer diurnal daylight lengths and high-angled, hotter solar rays to strike Earth during summer; whereas the tilt restricts our planet's winters to low-angled atmosphere-weakened rays. And also, a much larger hemispheric surface area is covered by diurnal sunshine during summer than during winter.

Finally, Kepler also discovered that orbital velocities of planets change inversely with their distances from the sun, so that Earth's average orbital velocity of 66,000 mph increased to its maximum of 67,000 mph on Jan. 2's perihelion and will decrease to its minimum velocity of 65,000 on July 6's aphelion. In essence, planets (and satellites which also have elliptical orbits, increase their orbital velocities while moving from aphelion to perihelion, and decrease their orbital velocities while moving from perihelion and decrease their orbital velocities while moving from perihelion to aphelion. This follows a Newtonian principle known as Conservation of Angular Momentum. If you have a bit of trouble deciphering all this, the ensuing poem may be of some help.

July 6 is Aphelion Day. We're farthest from the Sun.

Ninety-four point five times million miles; our distance from that one.

Earth orbits at its slowest speed - but still we'll have some fun.

Ellipses mark Earth's orbit path. Our course is never run.

Now half way round the orbit path - From farthest to the Sun,

In six month's time past Aphelion, we're nearest to that one.

Then Perihelion is the Day. And speed is Number One.

Nine-one point five times million miles - Close distance to the Sun.

Old Kepler had this figured out in Sixteen-Ten less one.

When all is said, and all is done, there's little new beneath the Sun.