Earth-Moon-Sun: Scale, Eclipses and Tides
Putting the Solar System to Scale
Before we can understand eclipses and tides we need to appreciate the vast differences in size and distance between Earth, Moon and Sun. The Sun has a mean diameter of approximately 1.4 million km — about 109 times wider than Earth. The Moon, with its mean diameter of ~3 500 km, is only about 1/400th of the Sun’s diameter. Yet by a remarkable cosmic coincidence the Moon is also roughly 1/400th of the distance from Earth to the Sun. This near-perfect size-to-distance ratio means the Moon’s angular diameter as seen from Earth is almost identical to the Sun’s angular diameter (both roughly 0.5°). This coincidence is what makes total solar eclipses possible.
Sun diameter: ~1.4 million km. Earth diameter: ~13 000 km. Moon diameter: ~3 500 km. Earth–Moon distance: ~384 000 km. Earth–Sun distance: ~150 million km (1 AU). The Moon and Sun have nearly equal angular diameters (~0.5°) as seen from Earth.
Solar Eclipses: Three Types
A solar eclipse occurs when the Moon passes in front of the Sun as seen from Earth. Because both the Moon’s orbit and the Moon’s size are not perfectly constant, three distinct types of solar eclipse are possible.
In a total solar eclipse, the Moon is at the right distance (close enough in its elliptical orbit) that it completely covers the Sun’s photosphere. The sky darkens dramatically, the solar corona becomes visible, and totality can last up to about 7 minutes. Observers must be inside the Moon’s umbra — the region of total shadow where no direct sunlight reaches. The path of totality is only around 100–250 km wide, but millions of people in the surrounding penumbra (partial shadow) see a partial solar eclipse.
In a annular solar eclipse, the Moon is near apogee (its farthest point from Earth) and therefore appears slightly smaller in angular size than the Sun. It cannot completely cover the Sun’s disc, leaving a bright ring — the annulus — of sunlight visible around the Moon’s silhouette. A partial solar eclipse occurs in the penumbral region surrounding any solar eclipse.
The Moon’s orbital plane is tilted about 5° relative to Earth’s orbital plane (the ecliptic). Most new moons therefore pass a little above or below the Sun in the sky and cast their shadow into space above or below Earth. Solar eclipses only occur when a new moon happens near one of the two nodes — the intersection points where the Moon’s orbit crosses the ecliptic. A total solar eclipse happens somewhere on Earth about every 18 months, but any one place sees one only about once in 400 years.
Lunar Eclipses: Total and Partial
A lunar eclipse occurs when Earth passes between the Sun and the Moon, casting its shadow across the Moon’s surface. Earth’s shadow has two zones: the central umbra (complete shadow) and the surrounding penumbra (partial shadow).
During a total lunar eclipse, the entire Moon enters Earth’s umbra. The Moon does not disappear entirely — it often glows deep red or copper as Earth’s atmosphere refracts and scatters sunlight inward, redirecting mostly red wavelengths onto the lunar surface. This is the same Rayleigh scattering that colours sunsets. The progression of a total lunar eclipse is described by four umbral contact points:
- First umbral contact (U1) — the Moon’s limb first touches the edge of the umbra; the partial phase begins.
- Second umbral contact (U2) — the Moon is fully within the umbra; totality begins.
- Third umbral contact (U3) — the Moon’s limb first re-emerges from the umbra; totality ends.
- Fourth umbral contact (U4) — the Moon fully leaves the umbra; the partial phase ends.
In a partial lunar eclipse, only part of the Moon enters Earth’s umbra, and only a dark ‘bite’ appears on the Moon’s surface. Lunar eclipses — unlike solar eclipses — are visible from the entire night-side hemisphere of Earth simultaneously, which is why any given observer sees more total lunar eclipses than total solar eclipses during a lifetime.
Tides: The Differential Gravity Explanation
Many students make a critical error when explaining tides: they think of the Moon pulling the nearest ocean toward it like a magnet. This explains only one tidal bulge — but there are two high tides per day on Earth. To understand both bulges we need differential gravity.
This is one of the most common errors students make. A single Moon-pull can only produce one oceanic bulge on the near side. But the far side of Earth is further from the Moon and experiences a weaker gravitational pull than the solid Earth beneath it. From Earth’s frame of reference, the far-side ocean is effectively ‘left behind’, forming the second bulge. Earth’s rotation then carries any location through both bulges in roughly 24 hours, producing two high tides and two low tides per day.
The Sun also exerts a tidal influence on Earth’s oceans, roughly 46% as strong as the Moon’s despite the Sun’s far greater mass (its much greater distance reduces its differential gravitational effect). When the Moon and Sun pull in the same direction — at new moon and full moon — their tidal effects add together to produce spring tides: unusually high high tides and unusually low low tides. When the Moon and Sun are at right angles to Earth (at first and last quarter), their tidal pulls partially cancel, producing neap tides: smaller tidal range with less difference between high and low tide.
Spring tides occur at new moon and full moon (Moon-Earth-Sun aligned); neap tides occur at first and last quarter (Moon-Earth-Sun at 90°). Earth’s rotation carries each location through two tidal bulges approximately every 24 hours, producing two high tides and two low tides daily.