Question 1

When is the next ISS pass over {city}?

Accepted Answer

The pass time updates every hour. Check the card above for tonight's exact start time in your local timezone. The ISS orbits Earth every 90 minutes, so visible passes happen in short windows — typically 2–3 per night for a few nights, then a gap of several days.

Question 2

How long is the ISS visible as it passes overhead?

Accepted Answer

Typical passes last 1–6 minutes. Longer passes (4–6 minutes) happen when the ISS passes nearly overhead; shorter passes (under 2 minutes) are low horizon grazes. The duration shown is from first appearance above ~5° elevation to disappearance.

Question 3

What direction should I look?

Accepted Answer

The start direction shows where the ISS will first appear. Look for a bright, steadily moving point of light — it moves faster than any aircraft and doesn't blink. It moves from the listed start direction to the end direction over the course of the pass.

Question 4

Why isn't the ISS visible every night?

Accepted Answer

The ISS orbits at ~51.6° inclination, so it passes over different parts of Earth at different times. More importantly, it's only visible when the ISS is sunlit AND your location is dark — the overlap window shifts daily and sometimes falls entirely in daylight or deep night when the ISS is in Earth's shadow.

Question 5

What's the difference between a high pass and a low pass?

Accepted Answer

A high pass (above 60°) puts the ISS near overhead — very bright, crossing the sky in a wide arc, easy to see. A low pass (under 20°) grazes the horizon — dimmer due to atmospheric haze, brief, and often cut short as the ISS enters Earth's shadow. High passes are much more impressive.

Question 6

Can I see the ISS during the day?

Accepted Answer

Technically yes, during very high passes when the ISS is at its brightest (magnitude -3 to -4) and the sky is deep blue. But it's extremely difficult to locate without knowing exactly where to look. Night or twilight passes are far easier.

Question 7

How bright is the ISS compared to stars and planets?

Accepted Answer

The ISS can reach magnitude -4.0 on overhead passes — brighter than Venus and everything except the Moon and Sun. More typical passes are -2 to -3. For comparison, Sirius (the brightest star) is magnitude -1.5. A mag -3 ISS pass is unmissable.

Question 8

What is apparent magnitude?

Accepted Answer

Magnitude is the astronomical brightness scale. Lower numbers (including negatives) are brighter: mag -4 is very bright, mag 0 is moderate, mag +3 is barely visible with the naked eye from the city. The scale is logarithmic — each step of 1 is 2.5× in brightness.

Marginal ISS pass tonight in Boulder

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