Is Earth getting closer to the sun, or farther away?

The sun moves in such a predictable way across the sky that you might never suspect that its relationship with Earth is changing all the time. In fact, the average distance between Earth and the sun is not static year over year. So do we know if Earth is getting closer to or farther from the sun? And what forces are acting on our planet and our star to make this happen?

In short, the sun is getting farther away from Earth over time. On average, Earth is about 93 million miles (150 million kilometers) from the sun, according to NASA. However, its orbit is not perfectly circular; it's slightly elliptical, or oval-shaped. This means Earth's distance from the sun can range from about 91.4 million to 94.5 million miles (147.1 million to 152.1 million km), NASA says.

Still, on average, the expanse between Earth and the sun is slowly increasing over time. This growing distance has two major causes. One is that the sun is losing mass. The other involves the same forces that cause tides on Earth.

The nuclear fusion reactions that power the sun convert mass to energy, following Einstein's famous equation E = mc^2. Because the sun is constantly producing energy, it's also steadily losing mass. Over the course of the sun's remaining lifetime — estimated at another 5 billion years or so, according to NASA— models of how stars evolve over time predict the sun will lose about 0.1% of its total mass before it begins to die, Brian DiGiorgio, an astronomer at the University of California, Santa Cruz, told Live Science in an email.

Although 0.1% may not sound like a lot, "this is a lot of mass," DiGiorgio said. "It's about the same amount of mass as Jupiter." Jupiter, in turn, is about 318 times Earth's mass, according to the Exploratorium in California.

The strength of an object's gravitational pull is proportional to how much mass it has. Because the sun is losing mass, its pull on Earth is weakening, leading our planet to drift away from our star by about 2.36 inches (6 centimeters) per year, DiGiorgio said. But we shouldn't throw the sun a bon voyage party just yet.

"This is pretty negligible, especially compared to the normal variation in Earth's orbital distance that happens because of its slightly elliptical orbit — about 3%," DiGiorgio said.

Just as the moon's gravitational pull results in tides on Earth, so does Earth's gravity tug on the sun. This stretches the side of the sun that faces Earth, resulting in a "tidal bulge," Britt Scharringhausen, an associate professor of physics and astronomy at Beloit College in Wisconsin, wrote for Cornell University's Ask an Astronomer page.

The sun rotates on its axis about once every 27 days, according to NASA. Because this is faster than the 365 or so days it takes for Earth to complete an orbit around the sun, the tidal bulge Earth generates on the sun sits ahead of Earth. The bulge's mass has a gravitational pull associated with it, tugging Earth ahead on its orbit and slinging it farther from the sun, Scharringhausen noted. (A similar effect is leading Earth's moon to slowly drift away from our planet.)

However, these tidal forces have a very weak effect on Earth's orbit: They cause Earth to move about 0.0001 inch (0.0003 cm) away from the sun every year, DiGiorgio calculated.

Might Earth's growing distance from the sun influence Earth's climate?

"As the Earth moves away from the sun, the sun's light will become dimmer," DiGiorgio said. Given that Earth's distance from the sun may grow by 0.2% over the next 5 billion years, "this dimming corresponds to a 0.4% reduction of solar energy hitting the Earth's surface," he said. "This is relatively small compared to the normal variations in the sun's brightness that happen due to the Earth's elliptical orbit, so it's not much to worry about."

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