Latitude says how far north or south of the equator a place is, and longitude says how far east or west of the Greenwich meridian it is, both measured in degrees.
Every place on Earth can be named by two numbers. Together they work like the row and column of a giant grid drawn on the globe. A satellite tracker uses the same two numbers to tell you which point on Earth a satellite is over.
Latitude measures distance north or south of the equator. The equator is 0°. It splits Earth into northern and southern halves. Latitude rises as you move towards either pole and reaches 90° at each pole.
Lines of equal latitude are called parallels. NOAA notes that each degree of latitude covers about 111 km on the ground.
Longitude measures distance east or west of the prime meridian. The prime meridian, at 0°, runs through Greenwich, England. Unlike the equator, it is not set by nature; an international conference chose it. The line on the opposite side of Earth, at 180°, is the antimeridian.
Lines of longitude, called meridians, meet at the poles. So a degree of longitude is about 111 km wide at the equator but shrinks towards the poles. By 60° north or south it is down to about 56 km, and at the poles it is zero.
The ISS flies in an orbit tilted 51.6° to the equator (NASA). So the latitude shown under it never goes past 51.6° north or 51.6° south.
The station moves at about 7.7 km/s, or about 460 km every minute. Divide by 111 km per degree: its path covers about 4° of arc a minute. That is why the tracker's numbers change so quickly.
The same grid works on other worlds. ISRO listed Chandrayaan-3's prime landing site on the Moon as 69.367621° S, 32.348126° E.
The Latitude and Longitude cards show the point directly below the station; the latitude never goes beyond 51.6° north or south. Open the ISS tracker
Size of a degree. On a sphere of radius R, an arc of one degree is 2πR/360. With Earth's mean radius 6,371 km: 2π × 6,371 ÷ 360 ≈ 111.2 km. Along a parallel at latitude φ, the circle has radius R cos φ, so a degree of longitude is 111.2 × cos φ km. At φ = 60°, cos φ = 0.5, giving about 55.6 km — NOAA's "56 km".
From latitude and longitude to x, y, z. Satellite software works in Cartesian coordinates. On a sphere, a point at latitude φ, longitude λ and height h is
x = (R + h) cos φ cos λ y = (R + h) cos φ sin λ z = (R + h) sin φ
The reverse is φ = asin(z / r) and λ = atan2(y, x), where r = √(x² + y² + z²).
Great-circle distance. The shortest distance between two points on the sphere uses the central angle Δσ between them:
Δσ = 2 asin √( sin²(Δφ/2) + cos φ₁ cos φ₂ sin²(Δλ/2) )
and distance = R × Δσ (in radians). Example: two points on the equator 22.9° of longitude apart — the spacing between successive ISS equator crossings — are 22.9 × 111.2 ≈ 2,550 km apart.
A caution. Earth is not a perfect sphere: NASA gives an equatorial radius of 6,378.137 km against a mean of 6,371 km. Maps and GPS use an ellipsoid model, so the latitude a GPS receiver reports (geodetic latitude) differs slightly from the simple geocentric φ above.
The equator is at 0° latitude and the North Pole is at 90° N.
Meridians meet at the poles, so the parallels get smaller. A degree of longitude is about 111 km × cos(latitude) long.
From 51.6° S to 51.6° N.