Today at around 7 p.m., Earth will reach the closer of the two apsides of its elliptical orbit: perihelion. In other words, Earth is closer to the Sun tonight at around 7 PM EST than it is at any other time of the year. Earth’s mean distance from the Sun is about 150 million kilometers. To be more precise, it is 149,600,000 kilometers, or, in slightly more convenient terminology if you’re comfortable with scientific notation, 1.496 x 108.
This number (1.496 x 108) is only the mean value, though.* It’s derived by some fairly technical methods you can read about here, or on Wikipedia. The actual distance between the Earth and the Sun differs throughout the year, as the Earth sweeps around the sun in its elliptical orbit. The approximate distance at perihelion is only 1.47 x 108; at aphelion, it is 1.52 x 108, only about 5,000,000 km farther away. This difference, though slight in astronomical terms, is non-negligible. You can see its effect here, in the difference between the visible disc of the Sun at aphelion and at perihelion as illustrated in this composite photograph from NASA’s Astronomy Picture of the Day website:
The photos were taken in 2008.
*Numbers like this are one reason astronomers prefer to make up their own units. For example, 1.496 x 108 can also be written as 1 AU, or 1 astronomical unit. And 9.4605284 × 1012 km is much easier to write as 1 ly, or 1 light year. Heavy-duty astronomers, unlike us amateurs, prefer to use even larger numbers like parsecs (PC), the distance at which a hypothetical star would have a parallax of 1 second of arc (1/3600 of a degree). That is, the distance an object has to be for its parallax to equal 1 second of arc. 1 PC= 3.26 light years, 30,800,000,000,000 km (3.08 x 1013). Serious astronomers like this unit because (a) it’s huge, and (b) it’s the smallest unit at which a reasonable telescope can discern parallax in a star over the course of a year. The telescope, riding along on the planet Earth, moves through roughly 2 AU in 1 year. If a star is displaced by 2 arcsec over the course of that year, it can be assumed that it is 1 parsec away. [UPDATE: reworded the above to fix an error pointed out by the sharp-eyed Brian Tung.]
Amateur astronomers are reasonably content with the more evocative light year, though, since it’s easier to understand the distance light travels in a year than it is to understand the amount of angular separation discernable over the course of a year. Or maybe it’s because Han Solo sort of ruined the parsec for many of us, when he conned Luke and Obi-Wan into hiring the Millenium Falcon by claiming that it “made the Kessel run in less than 12 parsecs.” Either we have to believe that it takes 12 times 10 trillion kilometers to make the jump to hyperspace, or we have to believe that Han was a fast-talking con man who didn’t really think Luke and Obi-Wan had any idea he was talking BS… Latter-day apologists have constructed an elaborate fiction that rescues Han from this charge, though.
[Table=1]
Related Images:
