Science News

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.

Article: http://physicstoday.scitation.org/do/10.1063/PT.6.4.20170807a/full/?utm_campaign=8569100_Q+-+TWIP+7-11+August&utm_medium=email&utm_source=Physics+Today&dm_i=1Y69%2C53NYK%2CE1MRRJ%2CJJX9K%2C1

Sharing is caring - please share this with your friends:

If you like this content, you will love the Resonance Academy.

Resonance Academy logo

Complete this form and click the button below to subscribe to our Science News Digest

No SPAM. Ever. That’s a promise.

X