On Saturday night, a rare celestial coincidence will make the moon look especially bright. The moon will be at its closest position to Earth at the same time as it enters its full phase, resulting in the apparently enormous “supermoon.”
The moon’s distance from the Earth varies all the time – from 220,000 miles away to 254,000 miles away -- because it orbits our planet in an ellipse, not a perfect circle. The moon actually won’t be that much closer, but can appear bigger and brighter than usual.
As with every full moon, the supermoon will have a slight effect on the tides. The sun, the Earth, and the moon will be in a near-straight line over the weekend. The combined gravitational effects of the sun and moon will make high tides slightly higher and low tides slightly lower – though this effect will vary locally.
While the full moon’s direct gravitational effect on Earth is actually quite small, the moon influences the tides based on a differential gravitational effect – the difference between the force exerted directly beneath the moon and the force exerted on the far side of Earth opposite the moon. The moon’s pull weakens across the diameter of the Earth, creating a gravity gradient that stretches the Earth slightly.
“The body of the Earth is fairly rigid, so it does not stretch much, but the oceans are much more easily moved,” Larry Sessions wrote for EarthSky on Monday. “Thus the effect piles up water on either side of Earth, and these piles of water – created by the differential gravitational effect – are the tides. Note that, on average, the tidal effect is quite small. It raises tides only a few feet across an 8,000-mile-wide planet Earth.”
Keep in mind that this tidal effect is happening all the time – a supermoon just adds a very, very small kick to it. Nor does the supermoon really affect the energy balance of the Earth, since our planet stores huge amounts of internal energy inside its crust, NASA scientist James Garvin explained in a 2011 interview.
“Nonetheless, these supermoon times remind us of the effect of our 'Africa-sized' nearest neighbor on our lives, affecting ocean tides and contributing to many cultural aspects of our lives (as a visible aspect of how our planet is part of the solar system and space),” Garvin said.
What about a big moon’s effects on animals? We can’t say for sure that wolves are more likely to be howling this weekend, but they might be less likely to be out hunting. Prey animals are less likely to be out and about in the light of a full moon, and a 2006 study published in the journal Behavior Processes indicates that predators are likely to have a night in when the moon’s shining its brightest. Brazilian scientists fitted three maned wolves with GPS tracking collars and found the animals traveled significantly less on the night of a full moon when compared to the darkness of a new moon.
Bottom line: don't expect any major disasters or changes to come along with the supermoon. It's a pretty celestial event, but really just another step in the cosmic dance routine of our planet and its satellite.