Satellites in the Night Sky: What Those Moving Lights Are

You are looking up on a clear evening when one of the stars starts to move.

It glides silently and steadily across the sky, no blinking lights, no engine noise, and then gradually fades away in the middle of nowhere.

You have almost certainly just watched a satellite.

Satellites are among the easiest things to see in the night sky once you know how and when to look.

Here is why they shine, how to tell them apart from planes, meteors and other moving lights,

and how to know in advance exactly when a bright one will pass over you.

Why Satellites Shine

Satellites do not carry lights bright enough to see from the ground.

What you see is sunlight reflecting off their bodies, antennas and solar panels, just as the Moon reflects sunlight.

For a satellite to be visible, two things must be true at once: you must be in darkness,

and the satellite, hundreds of kilometers overhead, must still be in sunlight.

That combination explains nearly everything about when and how satellites appear.

A satellite's brightness depends on its size, its shape, how reflective it is,

how high it flies and the angle between you, the satellite and the Sun.

When to Look

The best window is the first couple of hours after sunset and the last couple of hours before sunrise.

At those times the ground is dark, but satellites in low orbits are still high enough to catch the Sun's rays.

As the night goes on, the region of sky that is still sunlit shrinks, and low-orbit satellites spend more of their passes in Earth's shadow.

Around midnight, relatively few are visible, except in summer at higher latitudes,

where the Sun never dips far below the horizon and satellites can be seen all night.

Satellites in much higher orbits stay sunlit longer but are too far away to see without a telescope.

How to Recognize a Satellite

A few telltale features set satellites apart from everything else moving in the sky:

• Steady light. Most satellites shine with a constant glow and no flashing colored lights.
• Constant speed and a straight path. They move smoothly along a steady line, without turning.
• Silence. They are far above the atmosphere, so there is never any sound.
• A few minutes to cross the sky. A typical low-orbit satellite takes several minutes to go from one horizon to the other, much slower than a meteor.
• A gradual fade. Many satellites dim and vanish partway across the sky as they slip into Earth's shadow.

Most satellites travel roughly from west to east,

but those in polar orbits cross from north to south or south to north,

so direction alone does not identify them.

Space Stations: The Brightest Regular Visitors

Large crewed space stations are the showpieces.

With their big solar panels, they can appear brighter than any star, sometimes rivaling Jupiter or Venus.

The International Space Station, assembled in orbit beginning in 1998 and roughly the length of a football field, is a well-known example.

A good pass can last several minutes, with the station rising from one horizon,

climbing high and either setting on the other side or fading into Earth's shadow.

Watching one and knowing people live and work aboard is a memorable experience, especially for children.

Flares, Flashes and Trains

Not every satellite glows steadily. Some have flat, mirror-like surfaces that briefly catch the Sun at just the right angle.

The result is a flare: the satellite brightens sharply for a few seconds, sometimes outshining every star, then fades back to near invisibility.

Spent rocket stages and inactive satellites often tumble as they orbit.

Their changing angle to the Sun makes them flash or pulse rhythmically, sometimes every few seconds,

which is a clear sign of a tumbling object rather than an aircraft.

You may also see a line of satellites moving one behind another like a string of pearls.

This happens shortly after some launches, when a batch of small satellites is released together.

Over the following days and weeks they spread out and climb to their final orbits, and the line disappears.

Other Moving Lights You Might See

Plenty of moving lights are not satellites. Here is how to sort them out:

• Aircraft have blinking red, green and white lights, often a flashing strobe, and you may hear engine noise. At twilight, very high aircraft can catch sunlight and leave glowing contrails.
• Meteors are streaks that last a fraction of a second to a few seconds, far faster than any satellite.
• Drones hover, change speed and change direction, which satellites never do.
• Sky lanterns glow a flickering orange and drift with the wind, often in groups.
• Rocket launches can create spectacular glowing plumes when exhaust high in the atmosphere is lit by the Sun during twilight, while the ground below is already dark.

One more trap: if you stare at a single bright star or planet in a dark sky, it can appear to drift or wobble.

This is an illusion called the autokinetic effect, caused by tiny involuntary movements of your eyes.

Check its position against a treetop or rooftop and you will see that it is not moving.

Predict Passes With a Tracking App

The easiest way to catch a bright satellite is to know when it is coming.

Satellite-tracking apps and websites use your location to list upcoming visible passes. For each pass, they typically show:

1. The start time and the direction where the satellite will appear.
2. The time and height of its highest point, usually in degrees above the horizon.
3. The direction and time where it will set or fade into shadow.
4. Its expected brightness, given as a magnitude, where lower numbers are brighter.

Many apps can send an alert a few minutes before a bright pass.

Head outside a little early, face the starting direction and use your hand at arm's length to judge heights;

a fist covers about 10 degrees of sky.

If you photograph the night sky, expect satellites to appear as straight streaks in long exposures; many photographers simply remove them when combining several frames.

Frequently Asked Questions

Can I see satellites during the day?

Rarely.

The brightest objects can occasionally be picked out in a deep blue twilight or daytime sky by observers who know exactly where to look,

but satellites are really a dusk, night and dawn sight.

Never scan the daytime sky with binoculars near the Sun, and never look directly at the Sun.

Why did a satellite disappear in the middle of the sky?

It passed into Earth's shadow.

The satellite is still there, continuing along its orbit, but it is no longer reflecting sunlight toward you.

The fade often takes a few seconds and may turn slightly orange first, as the last sunlight reaching it passes through Earth's atmosphere.

Can I watch satellites through binoculars or a telescope?

Binoculars make it easy to follow a bright satellite and can reveal fainter ones.

Telescopes are harder to use because satellites move quickly across their narrow field,

but experienced observers with tracking mounts can even make out the shape of large space stations.

Look Up at Dusk

Pick a clear evening, step outside about an hour after sunset and watch the sky for ten or fifteen minutes.

Chances are good you will see at least one satellite gliding among the stars.

Add a tracking app, and you can greet the brightest passes right on schedule, knowing exactly what you are watching and where it is headed.