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Let's assume a lunar eclipse starts at 2359 on 1 January and ends at 0004 on 2 January? In this scenario, which date will be considered for recording this eclipse? The date it started on or the date it finished on?

This scenario may never happen in real life but I am just curious to know what is the process of selecting a date to record the eclipse.

Aaron S
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  • A total lunar eclipse generally lasts several hours. https://eclipsewise.com/lunar/LEhelp/LEbasics.html – PM 2Ring Dec 30 '22 at 06:49

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The date of the eclipse is that of its maximum. So if the maximum is at 23:59:59, then it’s on 1 January, but if the maximum is at 00:00:01, then it’s on 2 January.

It will be “advertised” as taking place on 1–2 January, to avoid confusion.

Pierre Paquette
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    Also important to note, they use UTC time for this determination, so they will often fall on a different date locally. – Greg Miller Dec 30 '22 at 14:41
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For record-keeping, this eclipse would be listed as being on the 1-2 January. There is no real astronomical special treatment being applied here. Events (be they terrestrial or astronomical) that start on one date and finish the next day are listed as occurring over two days.

This is very common. Most lunar eclipses last several hours and so are very likely to occur over midnight for some observers.

As much of observational astronomy is a nighttime affair, and events such as occultations, eclipses, or alignments may last for hours (if not longer) it is very common to use the 1-2 notation to mean an event starting on the evening of the first and ending on the morning of the second.

James K
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