Time, Tides and Eclipses

At first there may seem to be very little link between time, tides and eclipses. After all, what could link the measurement of the passing of time, the change in water levels and the blocking of an astronomical body. The answer is simple: they all rely on the interactions between the Earth, the Moon and the Sun.

How do we measure time?

Whilst time for observers on Earth moves at essentially the same rate, there is more than one definition of a day. First is the solar day, the 24 hours taken for the Sun to complete successive crossings of a given point. The other definition is that of the sidereal day, the 23 hours and 56 minutes taken for the Earth to rotate once on its axis.

On top of this we have two types of time, apparent solar time, which is the time on a sundial, and mean solar time which is the apparent solar of an average of the Sun. The difference between these is known as the Equation of Time (EOT) and is equal to zero only 4 times a year.

How do tides work?

Tides are the raising and lowering of water on the Earth's surface by the gravitational pull of the Moon and the Sun. There are 4 each day (so one every 6 hours) and they can be between two different types - the spring tide and the neap tide. A spring tide is where the Sun and the Moon are on the same side of the Earth or directly opposite, whereas a neap tide is where the Moon is at 90° to the Sun. It is for this reason that spring tides are larger than neap tides as the sum of the gravitational pulls upon the water is greater.

Eclipses

An eclipse is the blocking of light from the Sun to either the Earth or the Moon by the Moon and the Earth respectively. The first case, where the Moon covers the Earth, is known as a solar eclipse. The second case, where the Earth covers the Moon, is known as a lunar eclipse. Both can be either total (where the entire astronomical body is covered) or partial (where only part of the astronomical body is covered). In both cases the shadow cast has a dark central umbra and a lighter surrounding penumbra.

James Gooding, 2017

All image credits go to NASA (and respective centers/universities)