July 5, 2013 | 4
Notice anything different today? Is the sun looking a little smaller? Or the Earth moving a little slowly?
Well, today is Aphelion Day.
Around 11 A.M. Eastern, Earth passes through the point in its orbit that is farthest from the sun, a point called the aphelion. After today, we pick up speed as we fall back toward the sun, swinging around the opposite end of our orbit in early January.
Imagine a ball ricocheting off a bat. A loud crack. The ball climbs. Arcing over second base, its ascent slows. Then, for a moment, the ball appears to hover. As if gravity got distracted for a second. At a single point in time, all vertical motion stops. And then, just as casually, the ball falls back to the outfield as gravity regains its senses.
The Earth is the baseball. And we’ve just reached the top of our arc.
Out here, at aphelion, we’re now 152,098,232 kilometers from the sun. Six months from now, we’ll pass through perihelion: the closest approach to the sun. Between aphelion and perihelion, the Earth-sun distance changes by almost five million kilometers. But what’s a few million kilometers among friends? While it’s large compared to human scales (you’d have to circle Earth 125 times to cover the same distance), it’s actually only a 3 percent difference.
And 3 percent is exactly how much smaller the sun appears today than it will at perihelion in January. But even a trained eye wouldn’t notice that discrepancy. You would need to line up pictures of the sun from Earth at these two positions to notice that, yes, the sun’s apparent size does change from now until January.
Even the seasons are oblivious to the change. The dramatic temperature swings from summer to winter are a product of Earth’s tilt, not its orbital path. Today, the northern hemisphere sees more direct sunlight and longer days. South of the equator, the sunlight hits obliquely and the days are shorter.
Whatever your season, enjoy the day. And wave to the sun. We’re on our way back for a closer visit.
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