During the roughly 92 minutes the ISS takes for one orbit, Earth turns about 23 degrees eastward, so the next equator crossing lands that much farther west.
Watch the ISS on a map for a few hours and you notice something odd. Each lap crosses the equator at a different place. It is not that the station changes course. Earth is turning beneath it.
The station's orbit stays almost fixed in space. ESA notes that the station appears to travel from west to east over our planet. Meanwhile Earth spins once, west to east, in a little under 24 hours.
NASA gives the ISS 90 to 93 minutes per orbit. In that time Earth turns by about 23°. So when the station comes back round to the equator, the ground beneath it has moved east. On the map, the crossing point lands about 22.9° farther west, NASA says.
Suppose the ISS crosses the equator heading north at 80° E, south of India. Using NASA's figure of 22.9° per orbit:
So for a while the station passes over or near India on neighbouring laps, then its tracks move away for many hours.
The ISS makes about 15.5 to 16 orbits a day — not a whole number. So the crossings do not land in the same spots at the same time each day. NASA says the tracks repeat approximately every 3 days. That is why the times at which the station passes over any one city keep shifting from day to day.
On the map, compare where two neighbouring loops of the ground track cross the equator — they are about 23° of longitude apart. Open the ISS tracker
The basic shift. Earth's rotation rate relative to the stars is
ω_E = 360° / 86,164 s ≈ 0.004178° per second ≈ 15.04° per hour
Over one orbit of period T, the longitude of the ascending (northbound) equator crossing moves west by
Δλ = ω_E × T
For T = 91.4 min, Δλ ≈ 22.9°. For T = 92.6 min (a 400 km circular orbit), Δλ ≈ 23.2°.
Day-to-day drift. With n orbits per day, the crossings in one day are spaced Δλ apart and wrap around the globe n times. If n = 15.5, then after 15 orbits (about 23.1 hours) the crossing is 15 × 23.2 = 348° west — that is, 12° east of the starting longitude, about 0.9 hours before the same time the next day. After 16 orbits it is 11° west of the start.
Approximate repeat. About 46.5 orbits fit into 3 days at n = 15.5, close enough to a whole number for NASA to call it an approximate 3-day repeat. The exact pattern depends on height, which changes as drag lowers the orbit and reboosts raise it.
A second, slower effect. The orbit plane itself also turns slowly over days (nodal precession), which moves the crossings a little further; it is covered on the orbital-precession page. The per-orbit shift of about 23° is almost entirely Earth's rotation.
Earth rotates about 15° per hour, so in 92 minutes it turns about 23° east under the orbit. The crossing therefore appears 23° farther west on the map.
The track would repeat every day, crossing the equator at the same longitudes at the same times daily.