How to see the 空间站 tonight — and why the pass times never repeat

Why it is not simply "up there every night"

The station is 400 km up and orbits every 92 minutes, so it crosses your sky several times a day. Almost all of those passes are invisible, for two independent reasons.

First, it has to be above your horizon — obvious, but it means roughly two usable windows per day, not one continuous presence.

Second, and this is the part people miss: it has to be sunlit while your sky is dark. The station shines by reflected sunlight. When it passes over the day side of the 地球 it is in daylight and so are you, so there is nothing to see. When it passes through 地球's shadow it simply disappears, mid-pass, often quite suddenly.

The only time both conditions hold is a couple of hours after sunset and a couple of hours before sunrise, when you are in darkness but the station, higher up, is still catching the sun. That is why passes cluster at dusk and dawn and why some weeks genuinely have none — the orbital plane precesses and the geometry drifts.

What a pass actually looks like

Not a meteor, not a flashing aircraft. A steady white point, brighter than any star, moving in a smooth arc across the sky over four to six minutes. No blinking strobe — aircraft have one, the station does not. If it is fading in and out rhythmically, it is a plane.

The most common surprise is how fast it goes. It covers the width of the sky in about the time it takes to read this paragraph twice. If you are late by three minutes you have missed most of it.

At the end of a pass the station often does not set behind the horizon at all — it fades out while still high up, as it enters 地球's shadow. That fade is the signature of a satellite rather than a plane, and it is worth watching for.

Where to look first

Do not look at the 峰值. Look at where it appears, and look a minute early. A pass that peaks in the south might rise in the west-north-west; if you are staring south you will miss the first half and spend the rest of the pass trying to catch up.

Our city pages give each pass as a rise point and a set point as compass directions — for example 西北 → 东北 — because "look north-west first" is the only instruction that works in real time. The 峰值 elevation tells you how good the pass is: 10° is a low skim along the horizon behind rooftops, 60° and above is unmistakable and often passes close to overhead.

Why the times move every day

A pass on Monday at 20:14 will typically be followed on Tuesday by one at roughly 20:07, then 19:59. The station's orbital plane is fixed in inertial space while the 地球 turns underneath it, so each successive evening the geometry arrives a few minutes earlier. After a week or two the good passes have slid into daylight and the evening window goes quiet until the pattern comes round again.

That is also why a table of passes from last week is worthless. Predictions are only good for a few days, and only if computed from current orbital elements — which is what our city pages do, from the latest Celestrak element set.

What kills a pass

• Low elevation. Below about 20° you are looking through the thickest part of the atmosphere and past every rooftop, chimney and tree. A 12° pass is technically visible and practically disappointing.
• Cloud. Nothing to be done. The station is bright; cloud is brighter.
• Twilight. A pass 20 minutes after sunset happens against a sky that is still blue, and the contrast is poor. A pass an hour later against a black sky is far easier.
• Light pollution. Surprisingly not fatal. The station is typically magnitude −2 to −4, comparable to 金星 at its best, so it survives in a city.

Seeing it twice in one evening

Because the orbit takes 92 minutes and the 地球 turns 23° in that time, a second pass about 92 minutes after the first is common — but it usually tracks much lower and further east or west. If the first pass was good, it is worth staying out for the second one, and worth checking the table rather than assuming it will be overhead again.

Why I built the table

Because the standard advice — "check a website" — is a form that wants your coordinates before it will tell you anything, and I wanted to be able to look at a city, see tonight's passes with a rise 方位, and go outside. That is all /cities/ is: the same SGP4 prediction, computed for thirty places, with the times already in 当地时间 and the passes filtered to the ones that happen under a dark sky.

If you want the full picture of an evening rather than just the station, the observing plan puts sunset, twilight, the moon and the planets on the same axis.