The geometry of an eclipse
A solar eclipse occurs at new moon when the Moon passes between Earth and the Sun. A lunar eclipse occurs at full moon when the Moon moves through Earth's shadow.
The Moon's orbit is tilted by about five degrees to Earth's orbit around the Sun. Most new and full moons therefore pass above or below perfect alignment. Eclipses happen only when the Moon is near an orbital node at the right phase.
Total, partial and annular solar eclipses
Inside the Moon's narrow umbral shadow, the photosphere is completely covered and observers see a total solar eclipse. In the much wider penumbral shadow, only part of the Sun is covered.
An annular eclipse occurs when the Moon appears slightly smaller than the Sun, leaving a bright ring. Despite the dramatic appearance, annularity is not safe to view without a solar filter.
Why lunar eclipses are easier to see
A lunar eclipse is visible from almost the entire night side of Earth at once. The Moon can spend hours crossing the penumbra and umbra, so observers do not need to stand inside a narrow path.
Lunar eclipses are safe to watch with the naked eye, binoculars or telescopes. Their copper colour during totality comes from sunlight filtered and refracted through Earth's atmosphere.
What to record for any eclipse
- The type of eclipse and the local maximum obscuration.
- The times of each contact in the observer's time zone.
- The altitude and azimuth of the Sun or Moon.
- Whether the event is above the local horizon.
- The scientific source and calculation date.
Scientific references
References used for eclipse dates, safety guidance and local calculations.