Expert: Tom Whiting - 7/10/2009

Question
QUESTION: Dear Tom: I'm an author, writing a novel set in 1975. I have done some research and I know that in that year, the summer solstice fell on June 22, not June 21. I know, too, that this change in dates occurs only every 400 years. Here's my question. Would the autumnal equinox have also fallen a day later than usual in 1975? Since my novel deals with some elements of science fiction, this is a very important face for me. (By the way, my novel is already contracted for and will be published in 2010.) Many thanks.

ANSWER: Hi Sandra,
Yes, and actually it occurs every 4 years...to the solstice dates, the equinox dates, the perihelion/aphelion dates, and all meteor shower dates. The reason is that the year before a leap year....the leap years being 1976, 1980, etc....2004, 2008, 2012, etc.  our calendar becomes 3/4 of a day behind the "true" astronomical date,
because we can't add 1/4 day every year.  So by the time the
year before a leap year, ie. 1975, 1979.....2003, 2007, 2011, etc.
our calendar is 3/4 day or about 18 hours behind, so those events mentioned above....will nearly almost automatically fall on the following calendar day.  
(Not always, but most of the time, and not to all of them. If the 'standard occurance' falls in the first 6 hours of the normal date, then it will occur during the last 6 hours of the normal date with the calendar falling behind by some 18 hours- thus no date
change due to this effect.)

Then on leap years, we throw in a February 29th, and all is right
for the next couple of years, until the calendar gets behind again;
it's that simple. (Notice for the Northern Hemisphere, the first
day of winter can be either Dec. 21 or Dec 22 for that same reason).

I know we always advertise and tell people that the famous annual
Perseid meteor shower maximum is the night of August 11 -early morning hours of August 12.  But on those singular years just
before a leap year, we have to remember to tell people to go outside
on the night of AUGUST 12th -and early morning hours of August 13th...for the same reason discussed above. But it's all our
calendar system, and not the Earth's revolution about the sun,
nor the intercept point of the meteors changing.

What does happen every 400 years, the Julian calendar gets farther
out of step by 3 days from the current Gregorian Calendar.
The reason for that is the Julian calendar originally added in 3 extra days every 400 years, so over the centuries, the Julian calendar was getting way ahead of the astronomical position of the Earth around the sun. So that's why Pope Gregory eliminated something like 10 days from the calendar in 1582...and initiated the "End of Century Rule" divisible by 400 for the years 1600, 1700, 1800, etc.  That means years 1600, 2000, 2400 ARE leap years, whereas years 1700, 1800, 1900, 2100, 2200, 2300...are NOT leap years, even though they are divisible by 4, but NOT 400, thus eliminating the '3 days every 400 years' problem that existed in the Julian system.  Perhaps that's what you were thinking of, with the magic number of 400 years.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

FOLLOW UP:
Yes, I checked on your information about the Summer Solstice in
1975 was June 22 at 0027 hours GMT, now since then it will always fall on June 21, or even June 20...probably due to the slow precession of the equinoxes.  But that was only the GMT time for that year...observers west of Greenwich England, like the eastern USA, the date and time would have been 2027 EDT hours on June 21st....so there's another factor I didn't consider...the actual calendar date depends on which longitude you live at....your June 22nd date only applied to half the world from Greenwich east to the International date line. Everyone west of 0 degrees longitude the date would have read June 21st.
See
http://www.nmm.ac.uk/explore/astronomy-and-time/time-facts/equinoxes-and-solstic
Clear Skies,
Tom

ADDITIONAL FOLLOW UP:
I couldn't find the exact time for your Autumnal equinox for 1975,
but by extrapolating from the chart, halfway down the page on
http://scienceworld.wolfram.com/astronomy/AutumnalEquinox.html

we are assured that the Autumnal Equinox (for the N. Hemisphere)
had to be September 23rd in 1975 (a year before a leap year). By using the years just before a leap year, puts it at or around 1600 hours UT on 9/23/1975.
You can see from this Autumnal equinox chart the 'effects' of what I was talking about above...notice how the insertion of a February 29 in the Leap years, changes the date to the previous day, just due to
our calendar and that 1/4 day that can't be added every year.
Clear skies,
Tom
AND STILL ANOTHER FOLLOW-UP:
Yes, it looks like, judging by the graphs at the top of that last
website I gave you, due to the precession of the Equinoxes, that
June 22 date in 1975 is the LAST time it will ever happen on that
date for a long, long time. Notice as the centuries go by, the dates for the various events get earlier and earlier.  By the time we go half-way thru one precession event (the total taking 25,800 years) and halfway is 12,900 years from now, DEC and January will be our 'summer months' in the N. Hemisphere, and June/July will be our winter months, and the north star will be Vega instead of Polaris, as our axis makes that slow circular precession around the pole. You can google "Precession of Earth's Axis" if you're not sure what I am
talking about.  So it looks like your June 22 UT date will not occur
again...not for 400 years, but not for nearly 26000 years from now,
because it will get earlier and earlier as the centuries go by.
Hope this helps,
Clear skies,
Tom

---------- FOLLOW-UP ----------

QUESTION:   Tom, your response was/is great! I've printed it out and will go through it several times more to be sure I've understood it all but it clarifies everything I needed to make my use of the 1975 solstice appropriate to my story.
  I am amazed at your followup re the rarity of that particular date. It helps my plot enormously.
  I'm assuming, from what you've said, that though the autumnal equinox will fall on Sept 23, it will not be a particulary rare occurence but will be a usual one, because 1975 is a year before a leap year. Have I got that right?
   Clear skies, indeed. That's a wish for all the world.
   Thank you again.
   Sandra  

Answer
Hi Sandra,
Sorry BUT.....I'm afraid you have the "rarity" wrong and incorrect....June 22 was the date for probably a hundred years PRIOR to 1975...and was quite common.  It's just that 1975 marked the end of that sequence (or era), just like our children or grand-children might live to see the ending of December 22, or September 23 as the precession circle slowly and steadily progresses around.  Just like our great-grandfathers saw the end of June 23 and Sept 24 as the "final" date for 'their' summer and autumn.
26000 years (the precessional cycle)  divided by 365 days equals about a one day change every 71 years or so (and one day equalsalmost exactly 1 angular degree of travel around the sun at the Earth's orbit...but not all equinoxes or solstices are at the same time of day or same sinosoidal wave sequence, so they all don't change (meaning one particular calendar day comes to an end) all at the same time or year.  
So sorry, it's not that unusual for one generation to happen to see a sequential day...coming to an end, or a new one just starting, like June 20 for the summer solstice...just starting now.  I'm not sure which year June 20 will start, but it has to, eventually.  Oh, I just looked it up in Guy Ottowell's "Astronomical Calendar 2009"  and it's in 2012 for the first June 20th date.
For the Autumn Equinox, he also says, "until 1931 it sometimes fell on September 24th"! So our grandparents saw that date come to an end (Sept 24, 1931).  
For the Spring or Vernal equinox, he goes on to say...."Until 2007 the spring equinox fell on March 21 or 20; now only on March 20; in 2044 it begins falling sometimes on March 19th."  ---
So you could conceiveably write a sequel that 2007 was the "ending date" for March 21 at the Vernal Equinox too.  So see, it's not a rare thing, just the normal calendar and precession sequence, and certainly, not out of the ordinary after all.
For winter, he says, "from 1702 it has been on December 21 or 22; it will slip to December 20 in 2080".    
So see, all this slow date changing due to precession...is just a normal sequence of events...not rare, not out of the ordinary after all.  You can still "make it seem rare"  writing a fictional novel, but as you see above, it really isn't....Sorry about that.
Clear skies,
Tom Whiting
Erie, PA