Imagine a cool summer night in Northern Michigan. You’re camping less than 300ft from Lake Huron, it’s a New Moon, and the forecast is clear. You remove your tent’s rainfly, excited to sleep under the stars. You’re gazing up at the sky when a star appears to change its position. Are your eyes playing tricks on you? Do stars move? The answer isn’t as straightforward as you’d think.
Yes, your eyes are playing tricks on you – Your pupils need time to adjust to allow more light in. The longer you’re looking at the dark sky, the more your vision in low light conditions improve, making the quantity and brightness of the stars you see appear to be more significant over time. There’s also astronomical scintillation, which refers to observed changes in a distant star’s brightness or position. More commonly known as twinkling, this effect is caused by a star’s light being scrambled through our thick atmosphere as it travels to the Earth’s surface.
Yes, stars move – Everything in the universe moves due to electromagnetic and gravitational forces. Since a star’s light travels a massive distance to reach the Earth, its movement throughout the universe would need to be tracked over a much more extended period than a few minutes. You’re also always moving on the Earth in two ways, as it rotates on its axis and as it orbits the sun; just as our sun and moon rise and set as do the stars.
More likely, the moving star you spotted isn’t a star at all. There are over a million objects in Earth’s orbit. Of these, 2,666 are active satellites, and the rest are space debris or “space junk”, consisting of old spacecraft, lost equipment, and rocket boosters, among other things. While most of these items are hard to see, it’s possible you could spot a more massive object (over 30ft) without assistance. The largest and brightest of all human-made objects is the International Space Station. Traveling at speeds of 17,130-17,500 mph (roughly 5 miles per second!), the ISS orbits the Earth approximately every 90 minutes. If stargazing for a couple of hours, you could have spotted the ISS flyover!
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READ: 50 Things to See in the Sky
LISTEN: Sky Tour Astronomy Podcast
WATCH: Do Stars Move? Tracking Their Movements Across the Sky
Featured photo by Jongsun Lee from Unsplash