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Curated by RSF Research Staff

The Geometry and Physics behind the Great American Eclipse

On the 21st August at 10:30 am PST a total eclipse of the Sun will be visible across the northern regions of the United States of America, following a southeasterly path of totality from Oregon to South Carolina. Such an event – with a path of totality traversing from coast to coast (from west to east) – is a rarity, the likes of which has not occurred in nearly a century.

So why is such an event so rare?

At first glance, one may think that the alignment of the Sun-Earth-Moon system happens all the time and this event is not really that special. However – this is not the case - there are many variables at play and the coming together of all these variables must be just right! This does not mean to say that eclipses can’t be predicted – it just means it is not as simple and periodic as one might think and something that astrophysicist, Fred Espenak, knows all too well.

The Sun-Earth-Moon system comes into alignment twice a month – once with the Moon passing between the Earth and the Sun which is when we observe new moon and potentially a solar eclipse. Then again when the moon passes the other side of the Earth – such that the Earth is at the center of the 3-body system – which is when we see full moon and potentially a lunar eclipse.






However, as the elliptical orbits of the Moon about the Earth and the Earth about the Sun both follow different planes of rotation – the alignment is not quite right, and the moon is either too high or too low. In fact, the alignment is only optimal for about 6 weeks a year, and is known as “eclipse season”.

Then there’s the effects of tidal forces – which are exerted on the moon by both the Sun and Earth – and result in the nodes (where the Moon’s orbit crosses the ecliptic) moving westwards by approximately 19 degrees a year such that eclipse season starts 19 days earlier each year. The combination of this yearly cycle along with the three lunar cycles (node-to-node; new-moon-to-new-moon and perigee-to-perigee) results in the Saros cycle of approximately 6585 days which is the periodicity between eclipses of the same geometry. The eclipse next Monday – lasting for 2 minutes and 40 seconds (a third of the theoretical maximum) – is part of the Saros 145 cycle which was last observed on August 11th, 1999.


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