Earth & Moon Dance Leading to Eclipses

The currency in the realm of the heavenly “clockwork” are its cycles. From the small scale to the large, cycles are everywhere. The moon revolves around Earth; Earth revolves around the sun; and the sun revolves around the galaxy. There are cycles composed of combinations of other cycles. This month we’ll unravel the cycles underlying the upcoming total solar eclipse in August.

As discussed last month, the moon’s shadow, cast on the ground by the sun, will race across the USA from coast to coast on August 21. The movement of the shadow is due to the moon’s orbital motion as it revolves around Earth.  The moon takes about a month to go around the earth. Hence the lunar origin of the word “month” from moon.  While our calendar months vary from 28 to 31 days, the moon’s orbital period is not as variable.  Still there are two kinds of lunar months in astronomy.

The moon’s “Sidereal” month of 27.3 days how long it takes the moon to complete one revolution of the earth, or 360°.  In the meantime Earth is revolving around the sun in its yearly cycle, dragging the moon along with it.  The lunar motion causes the Earth-moon geometry to change leading to sequence of lunar phases we observe during the month.  From New moon to waxing crescent, waxing gibbous, full moon, waning gibbous and waning crescent back to new moon.

Let us choose new moon as an arbitrary beginning point for a lunar Sidereal month. At the end of that month comes a surprise: After 27.3 days, even though the moon has traveled completely around Earth, the moon’s phase is still a couple of days from returning to new moon.  That is because Earth has traveled part way around the sun since the start of the lunar Sidereal month.   Thus, the second lunar monthly period:  The 29 ½ day “Synodic” month is the time between similar lunar phases – such as from one new moon to the next. During a synodic month the moon travels in a complete circle around the earth, plus some.

As far as Earth goes, it takes one Tropical Year averaging about 365 ¼ days to orbit the sun.  But our Gregorian calendar is based on the 365 day Solar Year. The result of this difference is what makes leap years necessary in which a day is added roughly every four years making for a 366 day solar year.

What do these cycles have to do with eclipses?  The answer is that when combined and multiplied they are an important tool for predicting upcoming and historical eclipses of the sun (solar) and moon (lunar).  It turns out that a sequence of some 223 synodic months is so useful for predicting eclipses that it has its own name, the Saros, whose period is 18 years 11 1/3 days.

Following the passage of a Saros period, the sun, Earth and moon align in roughly the same geometrical configuration. A given eclipse will repeat one Saros cycle later. Such sequences of related eclipses are assigned a Saros number.  Thus each eclipse is a member of some numerical Saros eclipse family.

The repeated geometry isn’t 100% perfect.  The result is that each successive eclipse shifts slightly in Latitude. A Saros family begins with an eclipse path touching ground near one of Earth’s poles.  As each eclipse in the Saros repeats, the path shifts until the last eclipse of the Saros family at the opposite pole.  A typical Saros family experiences five or six dozen eclipses lasting well over a thousand years.

Successive eclipses in a Saros family also shift westward in Longitude. Recall that the Saros period is not an even number of days. There is an extra 1/3 of a day.  This is why each successive eclipse in a given Saros moves west about 120° in Longitude.  As a result, after three successive Saros periods, the same eclipse will repeat at roughly the same geographical region.

The upcoming August eclipse is a member of the Saros 145 family.  The prior Saros 145 eclipse was in 1999, passing through Europe, East Asia and parts of the Middle East. Prior to that, the 1981 eclipse crossed the then Soviet Union and Siberia.

Three Saros cycles ago in late July 1963 this same eclipse crossed Canada and the extreme northeast USA.  I recall viewing this Saros 145 eclipse as a child in Nebraska at my Grandfather’s farm.  Although we were far from the central path of totality the resulting deep partial eclipse was easily observed. My Father made a projection viewing device from a shoebox that allowed the eclipsed crescent sun to be safely viewed.

Three cycles later, Saros 145 will provide a similar opportunity for families to share the eclipse with their families and friends this summer.  Like Nebraska in 1963, Maryland will experience a deep partial eclipse in August.

The Westminster Astronomical Society has a supply of specialized “solar glasses” available for viewing the eclipse.  They can be picked up at events between now and eclipse time.  See a list of their upcoming events on the web site at and click on the “Events” tab.

— Curt Roelle

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