Distância Sol-Terra (AU)

Earth–Sun distance in astronomical units

One astronomical unit (AU) has an exact definition of 149 597 870 700 m, which corresponds to the mean Earth–Sun distance. The real separation isn't constant, though. Earth's orbit carries an eccentricity of e = 0.0167, so the gap shifts over the course of a year. Around perihelion in early January, Earth sits near 0.983 AU; by aphelion in early July it has drifted out to roughly 1.017 AU. For a quick first-order estimate you can use r ≈ a·(1 − e·cos(M)), where M is the mean anomaly measured from perihelion. On average, sunlight takes about 8 min 20 s to cross that distance (499 s).

Applications

It feeds into astronomical ephemerides and observation planning, and gives you a quick sanity check against NASA's JPL HORIZONS service. It also turns up when designing interplanetary trajectories and launch windows, modeling solar irradiance at the top of the atmosphere (which swings ±3.4% across the year), and refining tidal-prediction inputs alongside the Moon's distance.

FAQ

Why is January closer if it's winter in the north? What drives the seasons is the tilt of Earth's axis, not how far away the Sun is. That 3.4% range in solar flux barely registers next to the effect of the tilt.

How accurate is this estimate? For classroom purposes the Keplerian formula lands within about 0.5%. If you need precise ephemerides, you'll have to fold in perturbations from the Moon, the planets, and relativity.

Why 149,597,870,700 m exactly? Back in 2012 the IAU pinned the AU to that fixed integer, which freed the unit from any dependence on the gravitational parameter GM.