Last time, we said that Solar Eclipses and Lunar Eclipses usually come in pairs. For instance, the lunar and the solar eclipses occurred on 26 May and 10 Jun 2021 (not visible in Hong Kong) respectively are a pair. But why?

The paths of the Sun and the Moon on the celestial sphere cross each other at an angle of about 5 degrees and have two intersections. When both the Sun and the Moon move near the intersection at the same time, two bodies are almost on the same line in space, it is likely for the Moon to block the Sun seen on the Earth, i.e. a solar eclipse. Similarly, the Earth's shadow may cast on the Moon when the Earth is in the middle of the Sun and the Moon near the intersection, a lunar eclipse occurs.

The positions of the intersections (nodes) on the celestial sphere are not fixed but slowly shift westward. For example, 15 days after the lunar eclipse on 26 May, the Moon came in between the Sun and the Earth. A solar eclipse occurred on 10 June because they were still around the intersection. Another 15 days after the solar eclipse, even though the Moon orbited to the Earth's dark side again, a lunar eclipse did not occur because the Moon is away from the intersection. Similar for a solar eclipse. Therefore lunar and solar eclipses usually occur in pairs. Can you think of otherwise?