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Why Does a Lunar Eclipse Have Two Dates?

The date of a lunar eclipse depends on where you are in the world.

Map of Partial Lunar Eclipse on November 18–19, 2021

On our maps, a lunar eclipse is visible within the shaded areas.

©timeanddate.com

Same Moment, Different Local Times

A lunar eclipse plays out hundreds of thousands of miles away on the surface of the Moon. Observers on the nighttime side of Earth see the eclipse happen at the same moment.

Nevertheless, because observers are spread out across different time zones, their local times vary.

’Round Midnight, Before Dawn, and After Dusk

Lunar eclipses generally happen across two dates. As an example, let’s take the November 18–19, 2021 lunar eclipse, which we broadcast live. The map for this eclipse is shown above.

Our streaming partners in Hawaii were close to the point on the globe where the Moon was directly overhead at maximum eclipse. Here, the entire event was visible; it began before midnight, and ended after midnight:

  • Beginning: 20:02 (Nov 18)
  • Maximum: 23:02 (Nov 18)
  • End: 02:03 (Nov 19)

If we jump to the east—to our streaming partner in New York—the event began after midnight, and ended when the Moon set at dawn:

  • Beginning: 01:02 (Nov 19)
  • Maximum: 04:02 (Nov 19)
  • End (moonset): 06:57 (Nov 19)

If we jump to the west from Hawaii—to the Australian city of Sydney—the event began at dusk when the Moon rose, and ended before midnight. However, because we’ve crossed the International Date Line, this was the evening of the following day:

  • Beginning (moonrise): 19:34 (Nov 19)
  • Maximum: 20:02 (Nov 19)
  • End: 23:03 pm (Nov 19)

Two Dates

At timeanddate, we normally use two numbers to give the date of a lunar eclipse. Some sources give just one number, based on the time of maximum eclipse in UTC.

Fun fact: Not so long from now, there will be a total lunar eclipse that runs from one year into the next...

Topics: Eclipses, Moon, Astronomy