Earth  Sun  Planets and Moons  ID: 4711

2019 Total Solar Eclipse

On Tuesday, July 2, 2019, the Moon will pass in front of the Sun, casting its shadow across South America and the southern Pacific Ocean.

The Moon's shadow can be divided into areas called the umbra and the penumbra. Within the penumbra, the Sun is only partially blocked, and observers experience a partial eclipse. The much smaller umbra lies at the very center of the shadow cone, and anyone there sees the Moon entirely cover the Sun in a total solar eclipse.

In the animation, the umbra is the small black oval. The red streak behind this oval is the path of totality. Anyone within this path will see a total eclipse when the umbra passes over them. The much larger shaded bullseye pattern represents the penumbra. Steps in the shading denote different percentages of Sun coverage (obscuration), at levels of 90%, 75%, 50% and 25%. The images of the Sun show its appearance at a number of locations during the eclipse, each oriented to the local horizon.

The numbers in the lower left corner give the latitude and longitude of the center of the umbra as it moves eastward, along with the altitude of the Sun above the horizon at that point. Also shown is the duration of totality: for anyone standing at the center point, this is how long the total solar eclipse will last.

About Accuracy

You might think that calculating the circumstances of an eclipse would be, if not easy, then at least precise. If you do the math correctly, you’d expect to get exactly the same answers as everyone else. But the universe is more subtle than that. The Earth is neither smooth nor perfectly spherical, nor does it rotate at a perfectly constant, predictable speed. The Moon isn’t smooth, either, which means that the shadow it casts isn’t a simple circle. And our knowledge of the size of the Sun is uncertain by a factor of about 0.2%, enough to affect the duration of totality by several seconds.

Everyone who performs these calculations will make certain choices to simplify the math or to precisely define an imperfectly known number. The choices often depend on the goals and the computing resources of the calculator, and as you'd expect, the results will differ slightly. You can get quite good results with a relatively simple approach, but it sometimes takes an enormous effort to get only slightly better answers.

The following table lists some of the constants and data used for this animation.

Earth radius6378.137 km
Earth flattening1 / 298.257 (the WGS 84 ellipsoid)
Moon radius1737.4 km (k = 0.2723993)
Sun radius696,000 km (959.634 arcsec at 1 AU)
EphemerisDE 421
Earth orientationSOFA library iauC2t06a()
Delta UTC69.184 seconds (TT – TAI + 37 leap seconds)
ΔT69.368 seconds

A number of sources explain Bessel’s method of solar eclipse calculation, including chapter 9 of Astronomy on the Personal Computer by Oliver Montenbruck and Thomas Pflager and the eclipses chapter of The Explanatory Supplement to the Astronomical Almanac. The method was adapted to the routines available in NAIF's SPICE software library.

See this SVS page for a larger scale map of the eclipse path over South America that takes into account the lunar limb, Earth elevations, and other details that have been ignored here.


Visualization Credits

Ernie Wright (USRA): Lead Visualizer
Laurence Schuler (ADNET): Technical Support
Ian Jones (ADNET): Technical Support
Genna Duberstein (USRA): Producer
Ernie Wright (USRA): Programmer
Please give credit for this item to:
NASA's Scientific Visualization Studio

The Blue Marble data is courtesy of Reto Stockli (NASA/GSFC).

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Data Used:
Terra and Aqua/MODIS/Blue Marble: Next Generation
Note: While we identify the data sets used in these visualizations, we do not store any further details nor the data sets themselves on our site.

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