When the Earth, Moon and Sun Align

By Kevin Matyi

On Aug. 21, 2017, a total solar eclipse will cross the full continental United States along a narrow, 70-mile-wide path from Oregon to South Carolina.

The last total eclipse in the U.S. was in 1979. And the last total solar eclipse that crossed the entire continental U.S. happened in 1918. But why? Why has it been 99 years, and why have the intervening partial and even total eclipses caught only parts of the country?

In short, celestial geometry is complicated but predictable. Much like many other aspects of the cosmos, it is cyclic.

Need a minute to catch up? Go ahead. We’ll wait.  Credit: NASA

Eclipse cycles arise from a natural harmony between three motions of the moon’s orbit. We call them “months” due to their repetitive nature.

The synodic month governs the moon’s phases. It’s measured by the time it takes to go from one new moon to the next, which takes about 29 ½ days. In that time, the moon rotates once around its own axis and goes around Earth once.

From the perspective of a solar eclipse, the new moon phase is important. It’s the point in the moon’s orbit when it passes between Earth and the sun. A total solar eclipse can only happen at a new moon, and only when the other types of movement line up as well.

When the moon, on its orbit around Earth, reaches the point closest to the sun we can’t see the moon reflecting sunlight, so it appears dark. This is the new moon.
Credit: NASA/Genna Duberstein

New moons happen once a month, but we don’t see eclipses every month because the moon’s orbit is tipped by about five degrees from Earth’s orbit around the sun. On most months, the new moon casts its shadow either above or below Earth, making a solar eclipse a rare treat.

The moon’s tilted orbit meets the sun-Earth plane at two points called nodes. A draconic month is the time it takes the moon to return to the same node. The moon’s orbital nodes drift over time, which is why a single location on Earth’s surface might wait hundreds of years between total eclipses.

As the moon orbits Earth, it also wobbles up and down, making total eclipses rarer than they otherwise would be.  Credit: NASA

The moon’s path around Earth is not a perfect circle, which means the distance between us and the moon changes all the time. When the moon is closest to Earth in its orbit we call it perigee, and apogee when it’s farthest. This change in distance gives rise to the anomalistic month, the time from perigee to perigee.

The farther away the moon is from Earth, the smaller it appears. When the moon blocks all of the sun’s light, a total eclipse occurs, but when the moon is farther away — making it appear smaller from our vantage point on Earth — it blocks most, but not all of the sun. This is called an annular eclipse, which leaves a ring of the sun’s light still visible from around the moon. This alignment usually occurs every year or two, but is only visible from a small area on Earth.

When moon is too small to cover the entire sun’s disk, a ring or “annulus” of bright sunlight surrounds the moon.
Credit: NASA/Cruikshank

A total solar eclipse requires the alignment of all three cycles — the synodic, anomalistic, and draconic months. This happens every 18 years 11 days and 8 hours, a period known as a saros.

One saros period after an eclipse, the sun, moon and Earth return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur. The moon will have the same phase and be at the same node and the same distance from Earth. Earth will be nearly the same distance from the sun, and tilted to it in nearly the same orientation.

The extra eight hours is the reason why successive eclipses in the same saros cycle happen over different parts of Earth. Earth rotates an extra third of the way around its axis. Each total solar eclipse track looks similar to the previous one, but it’s shifted 120 degrees westward.

Earth turning on its axis impacts where total solar eclipses occur.
Credit: Espenak & Meeus

During this year’s total solar eclipse, anyone within the path of totality will be able to see one of nature’s most awe-inspiring sights. This path, where the moon will completely cover the sun and the sun’s tenuous atmosphere — the corona — can be seen, will stretch from Salem, Oregon to Charleston, South Carolina. Observers outside this path will still see a partial solar eclipse where the moon covers part of the sun’s disk. A total solar eclipse presents the rare opportunity to observe the corona and chromosphere, and eclipse observations are important for understanding why sun’s atmosphere is 1 million degrees hotter than its surface.

For more information on the eclipse, where to view it and how to view it safely (wear eye protection!), visit https://eclipse2017.nasa.gov/sun.

Make sure to wear eye protection when you go out to look at the eclipse!
Credit: NASA/Bill Ingalls


6 thoughts on “When the Earth, Moon and Sun Align”

  1. Excellent article, but I’m confused as to how the magnitude of the August 21, 2017 eclipse was calculated. According to several sources, that magnitude will be 1.0306, but I can’t find any place where the magnitude will be that high on August 21. For example, the eclipse’s greatest extent will be in Kentucky near Hopkinsville, but the magnitude there will be only 1.015, which is consistent with my rough calculation as to the relative earth-moon and earth-sun distances at the time of maximum eclipse. What is the specific formula that resulted in the 1.0306 number? Thanks

    1. Hi –
      We took your question to Dr. Alphonse Sterling, a solar physicist here at Marshall and here is his answer:

      Our Blog friend might be using the distance from the *center* of the Earth to the Moon,
      rather than the distance from the *surface* of the Earth to the Moon.

      Using values for the Earth-Moon and Earth-Sun distances from this page:

      I obtain the following:

      1) Using the geocentric distance from the Earth to the Moon at 18:00:00 UT on 21 Aug 2017, I get:

      Diameter of the Moon, as seen from the Earth’s center
      = D_M_EC = (arctan(3474./372822.)*180/!pi)*60.= 32.0324 arcminutes.

      2) For the same date and time, but using the distance from Earth’s *surface* (nearest Moon), I get:

      Diameter of the Moon, as seen from Earth’s surface
      = D_M_ES = (arctan(3474./(372822.-6371. ))*180/!pi)*60.= 32.5893 arcminutes.

      3) For the same date and time, the values for the diameter of the Sun are:
      = D_S_EC = (arctan(1.3914e6/151318102.)*180/!pi)*60.= 31.6099 arcminutes.

      4) and:
      = D_S_ES = (arctan(1.3914e6/(151318102.-6371.))*180/!pi)*60.= 31.6112 arcminutes.

      So (3) and (4) are about equal (as expected due to the Sun’s great distance), so we can
      say that D_S_EC=D_S_ES=D_S_E = 31.61 arcminutes, to two places beyond the decimal.

      For the magnitude, we want the ratio of the Moon’s diameter to the Sun’s diameter:

      Now, D_M_EC/D_S_E = 32.0324/31.61 = 1.01

      and D_S_ES/D_S_E = 32.5893/31.61 = 1.03

      We hope this helps! Thanks for the great question.

      1. Thanks very much for the detailed explanation — makes perfect sense. By the way, I have been away on vacation, and so did not see the response until today. A month from today, I will be in Hopkinsville to view the eclipse!

  2. I watched the solar eclipse from the exact place in Oregon in Feb 1979 and Aug 2017. In 38 years my memory may not be correct, but 1979 seemed to be darker during totality. Was the moon closer to earth for the 1979 solar eclipse than in 2017?

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