Aphelion 2013

If the sun looks a little smaller than usual as you're barbecuing this Fourth of July, it's not your imagination: Earth will be farther from the sun on Monday than on any other day this year.

That's because the orbits of all the planets in our solar system—including Earth's—are not perfectly circular, a phenomenon that was first explained in mathematical detail by the 17th-century German astronomer Johannes Kepler.

Kepler "figured out the orbits of the planets were elliptical in shape and that the sun was offset from the center," explained Mark Hammergren, an astronomer at the Adler Planetarium in Chicago, Illinois.

Earth's elliptical orbit means there will be a point each year when the planet is closest to the sun, called perihelion, and a point when it is farthest away, known as aphelion.