APOD: Julius Caesar and Leap Days (2016 Feb 29)

[APOD Robot]

Julius Caesar and Leap Days

Explanation: Today, February 29th, is a leap day - a relatively rare occurrence. In 46 BC, Julius Caesar, featured here in a self-decreed minted coin, created a calendar system that added one leap day every four years. Acting on advice by Alexandrian astronomer Sosigenes, Caesar did this to make up for the fact that the Earth's year is slightly more than 365 days. In modern terms, the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun -- actually we now know this takes about 365.24219 rotations). So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years. Eventually July (named posthumously for Julius Caesar himself) would occur during the northern hemisphere winter! By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Julian Calendar system was used until the year 1582 when Pope Gregory XIII provided further fine-tuning when he added that leap days should not occur in years ending in "00", unless divisible by 400. This Gregorian Calendar system is the one in common use today.

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[ems57fcva]

This APOD is slightly mistaken. The tropical year (364.24219 days) is not the time it takes the Earth to go around the Sun, but instead the time it takes the Earth to go from on vernal equinox to the next. The sidereal year is how long the Earth takes to go around the Sun, and its length is 365.25636 days. The difference is caused by the precession of the Earth's axis.

[Boomer12k]

Hail, Caesar.... oh, too soon? DEAD????? I didn't know he was sick...

Been cold and wet, and overcast mostly, but when clear enough, I have gotten out the Binoculars and looked at Orion and the Full Moon in the past week anyway.

:---[===] *

[ppdoc]

Does anyone know how the astronomers of 1582 were sophisticated enough to realize there were still seconds missing to the year? How would they have figured this out?

[Chris Peterson]

Does anyone know how the astronomers of 1582 were sophisticated enough to realize there were still seconds missing to the year? How would they have figured this out?

Not all that much sophistication is required. They didn't need to observe seconds per year, what they observed was a day or so per century. That is, they had centuries of observational data to work with. Even completely pretechnological societies worked out accurate lunar and solar calendars using nothing more than centuries of otherwise low precision observations.

[Guest]

Is that Cassiopeia and a crescent moon on the coin?

[Chris Peterson]

Is that Cassiopeia and a crescent moon on the coin?

That's Caesar on the side with the crescent. Venus is on the other side.

[beefcalf]

Chris, Our guest is referring to the sideways "W' above the lunar crescent. It looks like the constellation Cassiopeia.

[Guest]

Does anyone know how the astronomers of 1582 were sophisticated enough to realize there were still seconds missing to the year? How would they have figured this out?

Not all that much sophistication is required. They didn't need to observe seconds per year, what they observed was a day or so per century. That is, they had centuries of observational data to work with. Even completely pretechnological societies worked out accurate lunar and solar calendars using nothing more than centuries of otherwise low precision observations.

There was a book called "Calendar" published a few years back that gave the whole history of our current calendar but still glossed over how the actual ultimate calculation was done. Perhaps worth a read. Maybe.

[Rusty Brown in Cda]

The result is that every 28th year is identical, so that your 28th, 56th, 84th, 112th etc. birthday is on the same day of the week you were born and everything else that year is the same.
I assume this 28-year cycle is the result of 7 days a week x 1 leap year every 4 (i.e. the calendar can only begin on one of 7 days, but the leap year every 4th year throws this off).

[Steve Dutch]

Leap Year Day 2000 was the rarest scheduled event in human history, a leap year day in a century year occurs only once in 400 years. Leap Year Day 2000 was scheduled before the Pilgrims landed. And it slipped by with hardly any notice at all.

[Chris Peterson]

Chris, Our guest is referring to the sideways "W' above the lunar crescent. It looks like the constellation Cassiopeia.

Ah. I think that what we have is "PCM" with the "C" represented as a crescent. That would be for "P. Sepullius Macer", the moneyer who actually struck the coin. Also possible that the crescent isn't a "C" and doesn't represent "Sepullius", but either way, the "M" is, I think, just an "M" for "Macer".

[Rusty Brown in Cda]

Leap Year Day 2000 was the rarest scheduled event in human history, a leap year day in a century year occurs only once in 400 years. Leap Year Day 2000 was scheduled before the Pilgrims landed. And it slipped by with hardly any notice at all.

Maybe all the stress caused by the angst and panic a couple of months
earlier over the "Y2K" non-event exhausted us all such that we wanted
nothing to do with any such consideration for another 1,000 years.

[Tekija]

Does anyone know how the astronomers of 1582 were sophisticated enough to realize there were still seconds missing to the year? How would they have figured this out?

There must have been too many seconds to a year, because a few leap day additions were deducted beginning in 1585.

[neufer]

<<Julian day (e.g., 2,457,448 = 2³ x 7² x 6269) is the continuous count of days since the beginning of the Julian Period used primarily by astronomers.

The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar), a date at which three multi-year cycles started and which preceded any historical dates. For example, the Julian day number for the day starting at 12:00 UT on February 29, 2016, is 2,457,448. The Julian date (JD) of any instant is the Julian day number for the preceding noon in Greenwich Mean Time plus the fraction of the day since that instant.

The Julian Period is a chronological interval of 7980 years beginning 4713 BC. It has been used by historians since its introduction in 1583 to convert between different calendars. 2016 is year 6729 of the current Julian Period. The next Julian Period begins in the year 3268 AD.>>

---

<<Julienne is a tiny lunar crater that is located in Palus Putredinis (southeastern Mare Imbrium), in the irregular terrain to the southeast of the prominent crater Archimedes, and about 12 km west of the landing site of Apollo 15 at Hadley Rille.

This is a dumbbell-shaped feature, with many smaller craters around it. The surface about this crater is marked by ray material from the crater Autolycus to the northeast. There are narrow clefts in the surface to the north and south of Julienne, and a hilly region to the west.

Julienne is a culinary knife cut in which the food item is cut into long thin strips, similar to matchstick. Common items to be julienned are carrots for carrots julienne, celery for céléris remoulade or potatoes for Julienne Fries.>>

[Tekija]

Chris, Our guest is referring to the sideways "W' above the lunar crescent. It looks like the constellation Cassiopeia.

Ah. I think that what we have is "PCM" with the "C" represented as a crescent. That would be for "P. Sepullius Macer", the moneyer who actually struck the coin. Also possible that the crescent isn't a "C" and doesn't represent "Sepullius", but either way, the "M" is, I think, just an "M" for "Macer".

The crescent is the moon. It separates P M that stands for pontifex maximus, the highest religious leader. Caesar held that title when he renovated the calendar. On the other margin we have CAESAR IM, the latter stands for imperator, leader of the armies. The moneyer, or questor, was identified on the reverse or verso side, in this case L AEMILIUS BUCA, u spelled as V. Venus holds Victory in one hand and a sceptre in the other.

[Chris Peterson]

Chris, Our guest is referring to the sideways "W' above the lunar crescent. It looks like the constellation Cassiopeia.

Ah. I think that what we have is "PCM" with the "C" represented as a crescent. That would be for "P. Sepullius Macer", the moneyer who actually struck the coin. Also possible that the crescent isn't a "C" and doesn't represent "Sepullius", but either way, the "M" is, I think, just an "M" for "Macer".

The crescent is the moon. It separates P M that stands for pontifex maximus, the highest religious leader. Caesar held that title when he renovated the calendar. On the other margin we have CAESAR IM, the latter stands for imperator, leader of the armies. The moneyer, or questor, was identified on the reverse or verso side, in this case L AEMILIUS BUCA, u spelled as V. Venus holds Victory in one hand and a sceptre in the other.

Thanks. That makes better sense.

[neufer]

This APOD is slightly mistaken. The tropical year (364.24219 days) is not the time it takes the Earth to go around the Sun, but instead the time it takes the Earth to go from on vernal equinox to the next. The sidereal year is how long the Earth takes to go around the Sun, and its length is 365.25636 days. The difference is caused by the precession of the Earth's axis.

Does anyone know how the astronomers of 1582 were sophisticated enough to realize there were still seconds missing to the year? How would they have figured this out?

On vernal equinox the Sun rises in the east and sets in the west
and this provides an important indication for when to plant crops.

Calendars are often based upon some sort of sophisticated procedure
to count the number of days from one vernal equinox to the next.

If one's calendar count is wrong then sunrise & sunset shadows during the
supposed vernal equinox will turn up in the wrong places after a few centuries.

http://apod.nasa.gov/apod/ap140706.html
http://apod.nasa.gov/apod/ap100321.html
http://apod.nasa.gov/apod/ap090320.html

[stolenmoment]

Is it just me, or does Caesar look like a Kelly Freas drawing?

[MarkBour]

This APOD is slightly mistaken. The tropical year (364.24219 days) is not the time it takes the Earth to go around the Sun, but instead the time it takes the Earth to go from on vernal equinox to the next. The sidereal year is how long the Earth takes to go around the Sun, and its length is 365.25636 days. The difference is caused by the precession of the Earth's axis.

Thanks for pointing this out. It had to be a number less than 365.25, otherwise Gregory would have needed to add more leap days, instead of removing some of them. I also did not know that historically, as Art's Wikipedia post pointed out, it was especially the vernal equinox that was the most important to them. Makes sense that this was the one most directly used for planning the planting of crops, and that this would be a vital issue.

[Nitpicker]

This APOD is slightly mistaken. The tropical year (364.24219 days) is not the time it takes the Earth to go around the Sun, but instead the time it takes the Earth to go from on vernal equinox to the next. The sidereal year is how long the Earth takes to go around the Sun, and its length is 365.25636 days. The difference is caused by the precession of the Earth's axis.

I don't think the APOD description is mistaken to refer to the tropical year, which is indeed with respect to the Sun and the astronomical seasons it creates on Earth. The tropical year is what the various approximations applied to the calendar over the ages, were intended to match. The sidereal year is with respect to the fixed stars, so any calendar adjusted to it would see the seasons drift over time.

[Nitpicker]

Just for fun, I've made an accurate scale drawing of the (slightly) elliptical orbit of the Earth-Moon system around the Sun. Note that relative to the "fixed stars", the equinoxes and solstices rotate (opposite to the direction of orbit) about the Sun every ~26,000 years, or about 50 arcsec per year. The equinoxes and solstices define the Earth's seasons and the direction of the Earth's axial tilt.

The perihelion-aphelion line, known as the line of apsides, also precesses (along with the minor axis of the ellipse) about the ellipse centre, but much more slowly, taking ~112,000 years for a complete revolution relative to the "fixed stars". But relative to the equinoxes and solstices, the line of apsides processes (i.e. rotates in the direction of orbit) every ~21,000 years.

[neufer]

This APOD is slightly mistaken. The tropical year (364.24219 days) is not the time it takes the Earth to go around the Sun, but instead the time it takes the Earth to go from on vernal equinox to the next. The sidereal year is how long the Earth takes to go around the Sun, and its length is 365.25636 days. The difference is caused by the precession of the Earth's axis.

I don't think the APOD description is mistaken to refer to the tropical year, which is indeed with respect to the Sun and the astronomical seasons it creates on Earth. The tropical year is what the various approximations applied to the calendar over the ages, were intended to match. The sidereal year is with respect to the fixed stars, so any calendar adjusted to it would see the seasons drift over time.

• The APOD description IS mistaken in referring to the tropical year
  (364.24219 days) as "the time it takes for the Earth to circle the Sun."

"the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun -- actually we now know this takes about 365.24219 rotations)."

[Nitpicker]

The APOD description IS mistaken in referring to the tropical year
(364.24219 days) as "the time it takes for the Earth to circle the Sun."

I disagree. They are both around the Sun, but relative to different frames of reference.

I read "with respect to the Sun" as meaning as the Sun appears from Earth. It does not mean "with respect to the fixed stars".

[neufer]

The APOD description IS mistaken in referring to the tropical year
(364.24219 days) as "the time it takes for the Earth to circle the Sun."

I disagree. They are both around the Sun, but relative to different frames of reference.

I read "with respect to the Sun" as meaning as the Sun appears from Earth. It does not mean "with respect to the fixed stars".

APOD specifically states:

• "the time it takes for the Earth to circle the Sun"
  
  and not: the time it takes for the Sun to appear to circle the Earth.

The fixed stars simply define the Mach inertial frame of reference in which the action takes place.