The Longest Day

By Phil Plait | June 20, 2006 11:28 pm

At 12:26 Universal (Greenwich) time, or 5:26 a.m. Pacific time, the Sun will reach its highest declination in the sky (a celestial version of latitude). This is what astronomers call the summer solstice, and what Americans incorrectly call the first day of summer.

It means that at local noon (literally, halfway between sunrise and sunset), the Sun will be higher in the sky than it will be for the rest of the year, and every day until December 22 (the winter solstice) it will dip a bit lower. If you live in the southern hemisphere, than this means it’s the shortest day of the year, and the Sun will get higher every day, yadda yadda. Just reverse what I said above, or stand on your head if that helps.

Wanna know when the solstices and equinoxes are for the next couple of decades? High Hie thee to the United States Naval Observatory astronomical almanac. There’s lots more fun stuff there that’ll keep you amused, too.

Note added August 14, 2006: I was informed by a reader that it’s "hie thee", not "high thee". I’m an astronomer, not a middle English grammarian, Jim!

CATEGORIZED UNDER: Astronomy, Cool stuff, Science, Time Sink

Comments (38)

  1. Dave

    So when did GMT change to being called “Universal” time? And why? For someone who pays such attention to improper detail, I’d have thought you would prefer the more correct Greenwich name instead. At least that one is always correct, where I’m pretty sure someplace in this universe they aren’t in sync with this planet.

  2. PK

    Correct me if I’m wrong, but GMT is not the same as Universal Time. I believe GMT changes twice a year to incorporate daylight saving time.

    BA, that picture is awesome: Look at the size of that prominence! How many times can the earth fit in that loop?

  3. Al

    GMT remains GMT throughout the year, what may cause confusion is that from late March to October the UK runs on British Summer Time ie GMT+1

  4. Rob

    Sorry Phil

    Yet another example of the blatant northern hemisphere bias in astronomy ;) We are almost through Winter Solstice down here in Australia, not that we saw much of the Sun today anyway but at least the rain was welcome.

    By the way I tried standing on my head but apart from injuring my back the day was still short!

  5. PK

    So GMT became UT when the yankees realised Greenwich is not in the US? ;-)

  6. Stuart

    Actually GMT doesn’t change, the time used by the UK changes from GMT (winter) to BST (British summer time).

    Saying that GMT is no longer defined in its own right but is based on UTC (the civil version of universal time), which is within 1 second of UT1.

    GMT is still popularly used however.

  7. Evolving Squid

    Don’t we also hit aphelion around now?

    Ah the magic of google… July 3 around 2300 UTC is aphelion when we are farthest away from the sun.

  8. Most place I’ve lived, it’s typically warmer in September than it is in April, and it’s typically warmer in August than it is in May. Even though the strict and literal position of the Sun would suggest that June 21 is the *middle* of Summer, the typical temperature that’s out and about lags the position of the Sun.

    So, if you want to define seasons strictly by length-of-day, yes, June 21 should be the middle of the Summer (for the Northern Hemisphere). But if you want to define it by the typical temperatures you get, it’s a better approximation to say that June 21 is the beginning of Summer.

    -Rob

  9. Ruth

    Defining seasons by temperatures just doesn’t work because they change by year and location and global warming. You just can’t count on the weather. Length of day is the way to go AND around here it ties in nicely with traditional definitions (yes guys and gals it IS called Midsummer).

    So happy Midsummer / summer solstice / Litha / Midwinter / winter solstice / Yule to all! :o )

  10. Personally I feel that we have 12 months and 4 seasons, so it would make the most sense to define those seasons by months as a nice division. 3 months a season. This is how I “define” seasons in my own mind. Regardless of how factually accurate…

    Summer is June, July and August; Fall is September, October, November; you get the idea…

  11. BMurray

    Jesse, didn’t the French try that after one of their guillotine sprees?

  12. PK

    If I remember correctly, the periodic heating of a black body causes a periodic temperature fluctuation that is 1/8 out of phase with the heat source. This works both for days (it’s hottest around 3pm) and years (hottest in July/August).

    Unfortunately, I can’t find a solid reference to this result, and I am too lazy to rederive it.

  13. Isn’t this the day that we’re supposed to be able to stand a scrambled egg on its end?

  14. Sigh.

    Not all Yankees Americans people from the United States are complete morons, y’all. Thanks for remembering that. :)

    As for Coordinated Universal Time (UTC), here is a reasonably simple explanation, suitable for persons of (almost) all nationalities (by way of the links to articles in different languages over on the left sidebar). (Of course, if you can’t read English, my posting this won’t help.) :)

  15. Melusine

    PK,this one compares to about 35 earths, so they say. These shorter prominences aren’t so gargantuan (at least on the gargantuan-scale.) Either way, we’re all just tiny, tiny critters in the universe.

    For the 8-addicts, Spaceweather has a useful bit about analemmas today and here.

    I like saying Greenwich Mean Time, myself, rather than atomic or UT time, but I think for historical reasons.

    In Houston, Summer Solstice means that we can start scrambling eggs on our cars with regularity.

  16. Melusine

    I seemed to have lost a “here.” The “and” and the “here” are both two separate links that work. I hate typos. #-8~

  17. Rats! That is today? I was planning on waking up for the sunrise. Oh well, I will have to try again next year.

  18. Did something go wrong here. After I submitted my last
    comment, I got this message
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    Can you explain this, BA?

  19. Prowler67

    Don’t forget VMT (Verizon Mountain Time), that is what I use.

  20. J. D. Mack

    BA wrote Just reverse what I said above, or stand on your head if that helps.

    There, you see? Now it’s morning!

    Aw, he’s no fun, he fell right over!

    J. D.

  21. Melusine

    Speaking of clocks and time: It’s also Slow Motion Day.

    Gourd sees Slow Motion Day as part of a “slow movement” that says people ought to make the moment last, re-examine priorities and stop fighting the clock. The movement even has a book, In Praise of Slowness. It’s been described as a “fast read.”

  22. Now we have past the solstice here in the Northern Hemisphere, the winter nights are now drawing in and soon we will see the shops stocking up for the holliday season.

    Ho ho ho

  23. That bizarre error message has been cropping up on this blog for quite a while now … I posted three duplicate comments before I realized the comments were getting through…

  24. Maybe they wanted a name for the zeroeth time zone that was more universally acceptable to professionals across the planet – beyond just the English speaking bits. Greenwich Mean Time might also be Hiersac Mean Time, or Sanous Mean Time, or Yendi Mean Time, or Tema Mean Time (Tema sits closest to the Equator at nearly 0 Long. (31″ away). Better yet they could have defined the Prime Meridian at Entebbe, Uganda (on Lake Victoria) and gotten a spot practically on the Equator!

    Better yet! Wolf Volcano on Isabela Island in the Galápagos Islands. Just define 0 longitude since it straddles the equator.

    On the otherhand, maybe I’ll stick with Coordinated Universal Time.

    jbs

  25. Rumour Mongerer

    Head on over to http://www.spaceweather.com for a picture of the sun as it moves throughout the year (in a pattern called the “analemma”). Really neat looking.

  26. hale_bopp

    Meteorologists define winter as the three coldest months of the year and summer as the three hottest months. Spring and fall are in between, three month blocks each.

    Meteorological seasons start on the first of the month. Most places in the northern hemisphere, summer is June, July and August. You can work out the rest as I am feeling lazy about typing right now :)

    Rob

  27. Evolving Squid

    IMO:

    Spring starts when the last snow is gone from my front yard. This is typically late April where I live.

    Summer begins when my wife starts getting on about the lawn needing to be mowed. This is typically late May / early June where I live.

    Autumn begins when my wife gets on about the leaves needing to be cleaned up. This is usually sometime in October.

    Winter begins when I put snow tires on the car. This is usually mid-November.

  28. Miral

    Sounds like you’ve got a very short Spring then…

  29. Stuart

    I’m in the same position as Evolving Squid, Summer is the dry hot months of May, June and July, autumn is the wet months of August and September, winter is from first snowfall (mid-october) to last snow melt (mid-April) and spring is the few days in between.

    Typically we go from ski-ing (at -10C) to beach (+25C) in less then two weeks. Still its better than the UK where the summer is marked by warmer rain.

  30. PK

    Evolving Squid: do you have Snowdrops where you live? Then you can redefine the start of spring when they peek through the snow (February/March).

  31. Stuart

    PK: That made me laugh, sorry but we did have a particularly bad winter here this year. At the end of March the snow was about 4 feet deep, and I’d love to see the snow drop that could get through that.

    Joking aside the snow drops did start popping up in time for Easter. Even when we have little or no snow the frozen ground prevents them from coming in earlier.

  32. Nigel Depledge

    Oh, Phil, what a clanger!

    “High thee to the United States Naval Observatory astronomical almanac”

    Do what? Perhaps you meant “hie”, as in “I must hie me thither!”
    :)

  33. Jeff Corey

    The evil bastard is John Edward, not Edwards, even though Edwards is a lawyer, politicial and Democrat.

  34. icemith

    No one mentioned ‘ZULU’. That is also another name for GMT.

    Ivan.

  35. Maurice Dixon

    The firsr FULL day of Summer is June 22, and it is also my birthday. I have been telling people this for years.
    Any arguments?

  36. Mark Schindler

    Actually, that is not true for those of us here in the Hawaiian Islands, or elsewhere in the tropics. Twice a year, the Sun is directly overhead at local noon. This year, the dates are May 27 and July 16. Locally, this is called “Lahaina Noon”. In Hawaiian, Lahaina means “Cruel Sun”.

  37. Kaptain K

    It must be nice to have regular, definable seasons! ;)
    Here in central Texas, we have:
    1) Early summer, mid summer, late summer and next summer.
    2) Mild drought, severe drought, extreme drought and flood.
    3) Nine months of summer and three months of rough swimming! 0)

    Take your pick.

  38. steve garrett

    deja vu. if i spelled it right. what do you think. when i was five i had a dream of a house . it had two stories, a bird bath , and crayons in one room upstairs. i even asked my mom about it arround the time because i thought dreams where real. 14 years later in mount pleasent tx ,with my best friend jeremy, i saw the house in the dream . every detail was correct. i had never been in that town before and seen that house . well beside the dream i had at the age of 5. i believe the future exsists already and i my have an experiment that may trigger deja vu. i know it will but to what degree i dont know. even if it works we can not change what i would see or it would no longer be an accurate future. what do you think? how can you see the future if it doesnt already exsist?

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