The earth's orbit is slightly oval-shaped, so the earth is not always the same distance from the sun. In fact, the earth's closest approach to the sun, called perihelion, occurs today. As a result, our winters are actually a teeny bit milder than they would be otherwise.

That's because the date of perihelion does not remain fixed due to some complicated gravitational effects. So over time scales of thousands of years, the date slowly moves through the calendar, advancing by about one day every 60 years. For example, in about 1800 years, perihelion will be around February 2 instead of January 2, and in about 11,000 years, it'll be around the 4th of July.

This is more than an astronomical oddity. Though the changing distance from the earth to the Sun doesn't cause the seasons, it does have a small affect on temperature. For example, when perihelion is in July 11,000 years from now, our winters will actually be a little cooler than they are today. And interestingly, these slight changes have been linked to the comings and goings of Ice Ages.