Earth & Sun
The tropics
The Tropic of Cancer and the Tropic of Capricorn mark the outermost latitudes where the Sun can stand directly overhead at noon. Their position follows from Earth's axial tilt. Their names come from ancient astronomy.
Where can the Sun be directly overhead?
Move the slider through the year. The yellow point marks the latitude where the Sun is at the zenith at noon. It moves only between the two tropics.
Schematic drawing; sizes and distances are not to scale.
What is a tropic?
The Sun can be exactly overhead at noon only between about 23.44° north and 23.44° south. These two limiting latitudes are the tropics. At the northern tropic the subsolar point reaches its northernmost position; at the southern tropic it reaches its southernmost.
The whole region between them is called the tropics. The word goes back to Greek τροπή (tropḗ), meaning “turning” or “reversal”. τροπικός (tropikós) means roughly “connected with a turning”. It refers to the reversal of the Sun's apparent north-south movement.
Only within this zone can the Sun reach the zenith: once a year directly on a tropic, twice a year between a tropic and the equator. At the equator this happens around the two equinoxes. Outside the tropics, the Sun never reaches the zenith.
The underlying quantity is solar declination δ. Over the year it ranges between about +23.44° and −23.44°. This range corresponds to Earth's axial tilt relative to the perpendicular to its orbital plane.
Why are they called Cancer and Capricorn?
The names are historical. Ancient astronomers knew the constellations along the Sun's yearly path. Stars close to the Sun cannot be seen in daylight, so their positions were determined indirectly, for example from twilight observations and lunar eclipses. At the northern solstice, the Sun's position lay in Cancer; at the southern solstice, in Capricorn. The two latitude circles took their names from those constellations.
The Sun against the eastern zodiac
Move the date and latitude. Use the epoch switch to compare the constellations behind the Sun today with about 500 BC. The effect of precession becomes directly visible.
Compare today with about 500 BC: because of precession, different constellations lay behind the Sun at the solstices.
The stars directly behind the Sun are of course not visible in daylight. The constellations are simplified and shown in their relative position to the Sun. The historical view approximates precession since 500 BC; stellar proper motion and historical constellation boundaries are not reconstructed. During polar day or polar night there is no sunrise on that date.
Today the solstices no longer lie in front of the same constellations. The reason is precession: the direction of Earth's axis changes slowly. One full cycle takes about 26,000 years. The seasonal reference points therefore shift relative to the starry sky, while the old names remain.
Earth's axis does not point in exactly the same direction forever. It slowly traces out a cone. As a result, solstices and equinoxes shift relative to the constellations.
How could the Greeks know about the southern tropic?
They did not have to travel to 23.44° south. In Greek astronomy, a spherical Earth was an established model by classical antiquity. Once the yearly north-south movement of the Sun is measured, the southern tropic follows from the geometry of a sphere.
In principle, a shadow stick is enough. The Sun's altitude gives its declination. If the declination reaches about −23.44° at the winter solstice, the Sun must be at the zenith that day at latitude 23.44° south—even if the observer lives far to the north.
Could they see Capricorn at all?
Yes. Capricorn lies on the zodiac and is visible from the Mediterranean region. When the Sun itself is in front of it, daylight hides the stars, but its position relative to the star field can still be inferred. Hipparchus used lunar eclipses for this among other methods: the Earth's shadow on the Moon lies exactly opposite the Sun in the sky.