How Big Is That Supermoon Anyway?
Tonight’s “supermoon” is the biggest and brightest full moon of the year, due to the fact that the moon is near the closest point in its orbital path around Earth. But just how much bigger and brighter does it look? That’s a tricky question.
Most reports say the moon looks 14 percent bigger than usual, which is close to the truth but isn’t quite right. They also say it’s 30 percent brighter than usual, which isn’t right, either. James Garvin, chief scientist at NASA’s Goddard Space Flight Center, ran the numbers to come up with an explanation that seems to make the most sense.
First of all, it’s important to note that the moon itself is not getting significantly bigger or smaller. There’s a scientific debate over whether the moon is slowly shrinking or spreading out. But in either case, the change isn’t noticeable on human time scales.
The difference in the moon’s apparent size is basically a function of how close it is to Earth in its elliptical orbit. That orbit isn’t changing on human time scales, either. It just so happens that tonight, the moon is coming closest to Earth at the same time that it’s going full. Because the moon and the sun are precisely opposite each other, relative to Earth, tonight’s ocean tides may be a bit higher than typical — but again, the effect is nowhere near big enough to worry about.