GUIDES / BASICS
Why passes cluster at dusk and dawn
By Jeroen Wouda · 2 November 2026 · Basics
Anyone who checks a pass prediction list for a few weeks notices the same thing. Almost everything worth watching sits in the hour after sunset or the hour before sunrise. The middle of the night is empty, and the middle of the day is emptier.
This is not a scheduling quirk. It falls directly out of the geometry of a satellite at 400 kilometres and a planet turning underneath it.
Two clocks that have to agree
A satellite is visible only when two conditions hold at once. It has to be above your horizon, and it has to be in sunlight while your own sky is dark.
The first condition is generous. The station is above your horizon several times a day, at all hours. The second is what does the filtering. For roughly a third of each orbit the station is inside the Earth's shadow, and while it is there nothing on it is lit. The remaining two thirds it is lit, but during much of that time the sun is also up where you are, so the sky is too bright to see anything.
The overlap between lit satellite and dark ground is narrow.
Why the station is still lit after sunset
Stand at sunset and watch the sky go dark. The sun has dropped below your horizon, but it has not dropped below the horizon as seen from 400 kilometres up. The station is high enough to be over the curve of the Earth, still in direct sunlight, for a while after you have lost the sun yourself.
As the planet turns, the shadow of the Earth sweeps upward through the atmosphere. Your position enters it first. The station, hundreds of kilometres above you, enters it later.
The interval between those two moments is your viewing window.
The twenty degree rule
There is a rough way to put a number on this. Work through the geometry for an object at 400 kilometres and you get something close to 20 degrees: while the sun is less than about 20 degrees below your horizon, a satellite passing overhead can still be in sunlight.
That maps neatly onto the stages of twilight. Civil twilight ends when the sun is 6 degrees down, nautical twilight at 12, astronomical twilight at 18. So the usable window opens at sunset and closes somewhere near the end of astronomical twilight, when the sun reaches about 18 to 20 degrees below the horizon and the last of the scattered light has gone.
Inside that window the sky gets steadily darker while the station stays lit. The best passes are usually in the later part, when the sky is genuinely dark but the station has not yet slipped into shadow.
Before sunrise the sequence runs backwards. The station emerges from shadow while your sky is still black, crosses over, and then fades as the sky brightens ahead of the sun.
Why the window moves through the year
The orbital plane of the station keeps roughly the same orientation in space as the Earth travels around the sun. What changes, month by month, is where the sun sits relative to that plane.
In some weeks the plane is angled towards the sun. The station spends little or no time in shadow, and the passes land in the evening. A few weeks later the geometry has rotated, the station's shadow crossings fall at different points in its orbit, and the evening passes arrive later and lower until they stop being visible at all. The good passes have moved to the morning.
This is why the season rolls the way it does: evening passes for some weeks, then a gap of nothing, then morning passes, then a longer gap, then evening again.
The weeks when the station never sets
There is an exception worth knowing. When the orbital plane is tilted strongly towards the sun, the station can stay in sunlight for its entire orbit. It never enters the Earth's shadow, so it can be visible at any hour of the night, including ten or eleven in the evening and, at high latitudes in summer, through the small hours.
During those weeks the usual advice about twilight stops applying. A listing showing a pass at 23:40 may be perfectly good. What matters then is not the time but the elevation and the direction.
Summer at high latitudes
The opposite failure happens in midsummer. North of about 50 degrees, June nights never get properly dark. The sun dips a few degrees below the horizon and comes back up. The sky sits in a permanent deep twilight, bright enough to drown a satellite that would be obvious in September.
So the station can be overhead, in sunlight, at a perfectly reasonable hour, and still be invisible. This is not a missed prediction. It is the sky being too bright.
What to do with this
Two practical habits follow.
First, when you look at a prediction list, sort your attention by time before anything else. A high pass at 21:15 in the right season is worth planning around. A high pass at 01:30 is usually a shadow crossing, unless you are in one of the all-night weeks.
Second, do not conclude the station has stopped flying when the list goes quiet. It is still up there, fifteen times a day. It is simply passing over a dark planet, or over a lit one. The next season is a few weeks away.
Where to go next
- ISS passes shows which of tonight's passes fall inside the good window.
- How often does the ISS pass over my city? explains how the seasons fit together over a year.
- Tiangong: how to see China's space station applies the same geometry to a different station.
More guides
- Best time of year to watch the ISS — There is no single best month. The good weeks depend on the angle between the orbit and the Sun, and here is how that cycle works.
- How often does the ISS pass over my city? — The station circles Earth 15 times a day, yet you can go weeks without a visible pass. Here is why the schedule behaves that way.
- How to photograph the ISS — A moving point of light, a dark sky and a camera, and what it takes to turn a pass into a photograph worth keeping.