The earth has seasons because of a complex interaction between its distance to the sun (a minor influence), the tilt of the earth, the wobble of that tilt (known as "precession"), and shape of the orbit, which is elliptical. The earth reaches its closest point to the sun-- perihelion-- around January 3 (although this does vary). Aphelion, the farthest distance from the sun, occurs right around July 5. You may have also heard a couple of similar terms: apogee and perigee. Technically, apogee and perigee refer to earth's satellites, such as the moon; aphelion and perihelion deal with objects that orbit the sun.

You will notice that dates for aphelion and perihelion do not coincide with the solstices they are close to two weeks later. At solstice, the North or South Pole is pointing as much as it can toward the sun, but that doesn't mean that the earth has to be at its closest or farthest point ... they happen to be close together right now, but that will change over the millennia.

To add to the confusion, the earth is moving faster at perihelion than at aphelion, so technically, winters are a few days shorter in the Northern Hemisphere than in the Southern. Also, due to the fact that it takes a while for the earth to store up or give off heat, the actual coldest and warmest times of the year are usually a month or more following the solstices; this is known as heat lag, and it is even more pronounced for water than for land. This is why we don't go in the North Atlantic here in New Hampshire at least until July.

Thanks today to Kate Perry of the Christa McAuliffe Planetarium for her astrophysical expertise. The Weather Notebook receives funding from the National Science Foundation, and Subaru of America.