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Accuracy of Eclipse Times

Our eclipse pages show the start and end times for solar and lunar eclipses for millions of locations worldwide over several centuries. How accurate are the predictions?

21 August 2017 — Great American Eclipse (Total Solar Eclipse)

August 21, 2017 total solar eclipse path.

The total solar eclipse on August 21, 2017 was visible in many locations in the USA.

Consistent with NASA

First things first: yes, you can trust our eclipse times.

If you are planning to watch a solar or lunar eclipse, our eclipse pages are a great place to get reliable information. We have tested the information we display, compared it with sources like NASA and the Astronomical Almanac—and tested it again. You will not miss out on an eclipse because our times are off.

So why choose us? While our eclipse times are highly consistent with all of the most reputable sources in the field, we provide more service. Our eclipse maps, animations, and eclipse library simply make eclipse data easier and more fun to use.

Total Accuracy Is Impossible

Eclipse times can be determined mathematically. You only need to take into account factors like the size of the Earth, the Sun, and the Moon, as well as their movements in relation to one another, do the math—et voilà: the result will tell you when an eclipse starts, reaches its maximum, and ends at a location of your choice. But only just. There are some factors that make it impossible to be 100% precise. They include:

  • Our calculations do not take elevation into account; they are based on sea level for each location. If you are not at sea level, this means that the Moon's shadow falls a little bit closer to the Sun than shown on our Eclipse Map. For example, if the Sun appears in the eastern sky, the shadow's actual position will be a bit farther east. The higher your altitude and the lower the Sun's position above the horizon, the larger the difference.
  • Neither the Moon nor the Earth is perfectly round; they are both slightly squashed. Their exact shapes can only be determined up to a certain degree of precision. This means that the numbers any eclipse prediction is based on are merely approximations.
  • There are mountains and valleys on the Moon, so its shadow has a slightly rugged edge. Solar eclipse predictions usually do not take this factor into account. If you happen to stand in the shadow of a lunar mountain at the beginning or end of a total solar eclipse, totality may last a tiny bit longer for you.
  • The mountains and valleys on Earth further distort the timing of a solar eclipse. Our calculations are based on the size of the Moon's shadow at sea level. However, its umbra, the portion of the shadow where the Sun appears completely eclipsed, is cone-shaped. As a consequence, the higher you are above sea level, the larger the shadow—and the longer totality lasts for you.
  • The movements of the Earth and the Moon are not constant. While we know that the Earth revolves around the Sun in about 365.24219 days and the Moon completes a full orbit around Earth in relation to the Sun in about 29.530575 days, it is impossible to predict their precise speeds on a daily basis as a number of forces can speed things up or slow them down. This means that a day on Earth is rarely exactly 24 hours long and that the Moon's shadow may move slightly faster or slower during a solar eclipse than predicted.
  • We don't know exactly how large the Sun is. Even NASA wouldn't be able to tell you the exact diameter of our star! The margin of error is only about 0.03%, but that it is enough to influence the times for a solar eclipse by a few seconds as the size of the Moon's shadow also depends on the size of the Sun.
  • The Earth's atmosphere blocks some of the sunlight, so our planet's shadow has a slightly fuzzy edge. During partial or total lunar eclipses, this makes it difficult to determine the precise moment when the shadow first touches the lunar surface, which marks the beginning of the eclipse.

Very Small Margin of Error

So, how accurate are our eclipse times? It's difficult to put a number on it as any potential deviation depends on the type of eclipse, its date, your location, and other factors. But unless you need to know the moments an eclipse starts, ends, or reaches its maximum down to the second, it's fair to say that our predictions are so precise that, if there is a deviation, it is very small and you will most likely not notice it.

The factors mentioned above, while making it impossible to be absolutely precise, only cause minute distortions. For example, the fact that the lunar shadow increases in size as you move towards the Moon creates a slightly longer totality at higher altitudes. However, because the lunar shadow moves at such a high speed and its diameter increases at a very slow rate, this difference only amounts to fractions of a second. Even on Mount Everest, the world's highest mountain, the Moon's shadow is only about 80 meters (260 feet) wider than at sea level.

Similarly, the unpredictability of the Earth's rotational speed, while having the potential to significantly distort the predictions of eclipses in the far future, poses less of a problem for the calculation of eclipse times in the next few years. The closer the eclipse is in time, the more reliable the calculated times are.

In terms of the location of the Moon's shadow during solar eclipses, the margin of error may sound pretty large at first. For example, the location of the Moon's shadow shown on the Eclipse Map can be off by about 10 kilometers (33,000 feet). However, that is the approximate distance the Moon's shadow covers in only 10 seconds, so this imprecision does not amount to much for spectators on the ground. Having said that, if you are at the very edge of the path of the total eclipse, it is probably wise to move at least 1 kilometer towards its center to make sure you will not miss out on totality.

Beware of the Horizon

It is worth noting that the times we display for sunrise and sunset, as well as moonrise and moonset, are based on a flat horizon where the observer is at the same altitude as the horizon. For this reason, it is important to avoid obstacles like hills or buildings if you are planning to watch an eclipse where the Sun or Moon appears close to the horizon.

Please also note that in rare cases, our list of eclipses seen in a particular location may not include a solar eclipse that occurs during sunrise or sunset. However, we are already in the process of tweaking our models to fix that issue.

Next eclipse begins in

141Days 6Hrs 8Mins 5Secs

Partial Solar Eclipse

Jan 5, 2019 at 23:34 UTCSee more

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Solar Eclipses

  1. When Is the Next Solar Eclipse?
  2. Different Types of Eclipses
  3. What Are Solar Eclipses?
  4. How Often Do Solar Eclipses Occur?
  5. Total Solar Eclipses
  6. Partial Solar Eclipses
  7. Annular Solar Eclipses
  8. Hybrid Solar Eclipses
  9. Solar Eclipses in History
  10. Solar Eclipse Myths
  11. Magnitude of Eclipses

Eclipses


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