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Universal Time (UT)

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Astronomers and scientists need to tell time in a way that is not possible to be misinterpreted. The time system used every day by regular people involves time zones, so It’s not enough to tell someone it’s 12 o'clock or 3 o'clock – because you have to specify AM or PM, and where on earth you are. When it’s 3AM in New Zealand, it’s definitely not 3AM in New York or Moscow or Buenos Aires. And to be honest, how many people in North America even know what the time zones are for other cities around the world? If you do business regularly with individuals in India, you‘d probably know what time it was there. But for most people, there would be really no need to know what each other’s time zones were – and that makes it difficult to talk about time in a way that is meaningful.

And how about airlines? How can we expect pilots to be able to use a system that offers “we’re taking off from Newark at 6PM, and we’re going to make an 18 hour flight to Singapore, so we’ll be landing at … uh… quick! what time will it be in Singapore when we land?” Now, if you knew that we’re going to take off at 6PM with reference to some standard time description that was the same for everyone – that is, “6PM” by this standard was the same for folks in both Singapore and Newark – then, it would be an easy task to figure that you’d fly your 18 hour route and land at “noon” the next day.

The 24-hour clock

And we can do better that that – because I don’t even have to say “PM” if I use the 24 hour clock. People in the USA aren’t too used to that, unless they’re pilots or scientists, or are in the military. In this case, I’d say I’m taking off at 1800 (which is 6PM on the 24-hour clock), and if I add 18 hours to that I get 3600. Subtract out the 24 hours that make up today and I see that it is 1200 – noon! It’s just gotten a whole lot easier to deal with time.

Make sure you’re comfortable with using the 24-hour clock, though: 1800 is 6PM, 1500 is 3PM, and 2300 is 11PM. All the AM times are easy, because they are the same in both systems: 0900 is 9AM, and 0630 is 6:30AM. For the PM times, just remember that 24-hour time is always obtained by adding 12 hours to the time you normally say, and don’t use the colon that you normaly do. So, for 7:35PM you add 12 to 7 and get 19, then drop the colon to get 1935. (We WILL use the colon when we are including seconds, so for example 4:30:15 in the afternoon will be 16:30:15UT in our tables.) For dead-on midnight, we use 0000 instead of 2400, because that instant (00:00:00UT) is the beginning of a new day.

Only 24 hours in a day!

If you ever do calculations with time, and end up with a time that is greater than 2400, then you just have to think that you’ve carried over into the next day. Subtract out 24 hours, and that’ll give you the time that the result should be – only it’ll then be "tomorrow" in that case.

We need to give a name to this time-keeping system that uses a universal time zone that isn’t different for everyone on Earth. How about we call it “Universal” time? Why not? If you want to call it a regular time zone name, you could say Greenwich Mean Time (GMT). If you’re in the military or aviation you could say Zulu Time (Z), but these are just different names for the same thing. (Please note that it is NOT necessarily the current time in London, because the UK observes Summer Time. One country that IS always on GMT, though, is… Iceland!)

Now, there are some intricacies of formal Universal Time (which we’ll call UT from now on), that are only of interest to scientists and astronomers. There is Coordinated Universal Time (UTC), and then there are UT0, UT1, UTR, and UT2. None of these variants means anything to normal people, though you can read about the different flavors of Universal Time if you’re interested. Just like astronomers have several different ways of measuring how long a month is, they have different ways of measuring time with very subtlely different shades of accuracy – none of which makes any difference at all as to when the eclipse is going to start on April 8, 2024!

Time conversions

One thing we are going to need is a way to translate UT to and from the time zones we’re used to using in everyday life. Why should we do that, when we went to all the trouble of making this cool system to begin with? Well, we have to stop short of talking with our friends about the eclipse, and telling them that it starts at “1813UT”. Normal people don’t talk like that (although, you’ll find that on a lot of places on, we’ve done exactly that…); normal people want to hear things like “2:13 in the afternoon”, so we should do the same.

(Why haven’t we done it on Well, frankly, it’s actually more difficult to do calculations in the normal time zones, and also, we can’t tell where you are located – so we can’t tell what time zone you’re in. We CAN tell what time zone a particular city is located in, so in some cases we’ve been able to gives times in local time. We have in EVERY case tried to be very careful to label any times we’ve given, so you know what time system or zone we’re talking about.)

Remember, though, that while UT is the same at every instant for everyone on Earth, it will convert into different times for folks in different time zones. The 2024 eclipse covers a lot of territory, and if you count the people who will be able to see the partial phases, it’s a whole lot of time zones. Plus, some places observe Daylight time and some don’t. (North America is VERY diverse when it comes to time reckoning!) We’ve gathered them all up, and here is a chart to show you the conversions:

Time Zone UT Offset Where Observed
GMT (UT) 0 (Standard UT time)
PMDT -2 St. Pierre and Miquelon
NDT -2.5 Newfoundland (and a couple towns in Labrador)
WGST -3 Western Greenland Summer Time
ADT -3 Atlantic DT – Many of the Maritime Provinces
AST -4 Puerto Rico and the Caribbean
EDT -4 Eastern US and Canada
EST -5 Quintana Roo, and a handful of communities in Canada
CDT -5 Central US, Canada and Mexico
CST -6 Saskatchewan (the same as Alberta time – MDT) and Central America
MDT -6 Rocky Mountains of Canada, the US and Mexico
MST -7 Most of Arizona and Sonora
PDT -7 Western Canada, the US and Mexico
AKDT -8 Alaska (except the Aleutian Islands)
HST -10 Hawaii

This table represents all time zones in North America for which part of the 2024 eclipse is visible. (The eclipse is not visible at all in the Aleutian Islands; we’ve also omitted time zones for the Pacific Islands that will see the eclipse’s partial phases in their early morning hours.)

Do you see why it’s important to have one time standard for everyone when we do calculations? It’s complicated enough just to know what time zone you’re in!

So how do we use this to convert times? Take the UT that you see on (or anywhere else), and apply the “UT offset” shown above to get your local time. So, if you see 1315UT, and you’re in the CDT zone, subtract 5 hours to get 8:15am.

[If you’re going the other way, then you have to do the opposite operation: If you know it’s 7PM right now (1900 local), and you’re in Hawaii, then ADD 10 hours to get 2900 which then goes down (you have to subtract 24) to 0500UT the next day. The conversion in this direction isn’t one you’ll have to do for the eclipse, though.]

What time zone am I in?

How do you know what time zone you’re in? Well, the vast majority of places (except for the Caribbean and Arizona/Sonora) will be observing daylight time on eclipse day. You should know if you’re in a location that doesn’t. And if you get stuck, just do a web search for “Current Universal Time” and compare what it says to the time you see on your phone. If the UT is 4 hours ahead of your local time, then you know you’re in the EDT or AST zone! (Or if you want, download a UT clock app, and you won’t even have to do any conversions – though you may tend to be really early for appointments if you look at the UT clock by mistake!)

Daylight Saving Time

One final note: Most places that observe Daylight Time will switch back and forth in the Summer and the Winter. In all cases that change time like this, the eclipse happens during the part of the year that you’ll be on Daylight time. So if you change, and in the winter you’re on PST, and you don’t see “PST” in the table above, remember that’s because you won’t be on PST when the eclipse is happening!