Do You Mean Apparent?
Mark Pottenger
Are you missing something that should be apparent? We have recently had brought to our attention (largely because of the efforts of Arthur Blackwell) the need to use apparent (rather than mean) time for many charts before the 20th century.
It is easy to assume that any chart from before the introduction of time zones should be in Local Mean Time (LMT). After all, that is what the time change books imply. As Arthur Blackwell has been saying for some time (long enough to finally penetrate), LMT itself was only in use for about a century before time zones. Before LMT, LAT (Local Apparent Time) was the dominant form of timekeeping. LAT (or sundial time) truly keeps time by the sun, with all its unevenness of motion during the year.
Which is what?
Local Apparent Time is what you would read from an accurate sundial. It is noon when the sun is at its upper crossing of the meridian. Some days are longer than others (we all knew that anyway—especially about Mondays).
Local Mean Time takes all the unevenness of the sun’s motion in a year and averages it out (“mean” is another word for “average”) to produce an even measure of time. Every second, minute, hour, or day of mean time is the same length (aside from leap seconds).
Why?
The earth travels around the sun in an orbit that defines the plane of the ecliptic. Earth rotates on an axis tilted with respect to the ecliptic, defining the plane of the equator. As a result, the apparent motion of the sun in the sky caused by the actual motion of the earth is going in two directions at once. The slower yearly motion along the ecliptic is at an angle to the faster daily motion parallel to the equator. Near the equinoxes, when the sun is at the ecliptic-equator intersections, the angle causes the sun’s apparent ecliptic (yearly) motion to produce a slower apparent motion parallel to the equator. Near the solstices, when the sun is at maximum declination where the planes are closest to parallel, the sun’s apparent ecliptic motion produces a faster apparent motion parallel to the equator. Since the daily and yearly motions are both in the same direction (counterclockwise when viewed from the north), the effect of the sun’s ecliptic motion is always to lengthen the apparent day from what it would be if there was no motion (because the rotation has to go a little more than a full circle to catch up to a moving target). The result of this is that when the sun’s yearly motion projected on the equator is faster, the apparent day is longer and when the motion is slower, the day is shorter!
The preceding describes the cause of one main component of variation in the apparent length of the day—inclination, the angle between the ecliptic and the equator. The cause of the other component is eccentricity. The earth’s orbit is not perfectly circular, and the deviation from circularity is called eccentricity. When the earth is closer to the sun it moves faster than when it is farther out. The closest approach is called perihelion and currently falls in January and the farthest point is called aphelion and currently falls in July. Since one solstice falls in late December, that solstice and the perihelion reinforce each other’s effects to produce faster motion, while the June solstice and July aphelion partially cancel each other. The effect of the inclination is stronger than the effect of perihelion, so there is still some speeding up of the apparent motion of the sun in June and July, but not nearly as much as in December and January.
The actual difference between the length of any single mean and apparent day is always measurable in seconds—not even a full minute. However, the differences build up over many days. The difference between LMT and LAT at any given moment is called the “equation of time”. The equation of time is currently largest in early November, reaching about 16 1/2 minutes. The equation of time was originally used as a correction to get from LAT to LMT, because direct observations gave LAT. Since sometime last century, it has reversed its meaning: it is now the correction to get from LMT to LAT. Using it in the modern sense, the equation of time for 12 GMT on the first of each month in 1900 was (in minutes & seconds): -3:40 -13:47 -12:35 -4:02 2:57 2:27 -3:31 -6:08 0:0 10:14 16:19 10:55. The reason I specified what year I was giving figures for is that the equation of time is not very fixed. The inclination, the eccentricity, and the dates of perihelion and aphelion all change with time. All of these changes interact over centuries and produce changes in the apparent motion of the sun and in the equation of time.
(Aside: If you record the position of the sun once per day at the same mean time of day for a year, you get a distorted figure 8 shape whose width is the variation in time and whose height is the sun’s declination range. This figure is called an analemma. There is a beautiful poster-sized picture of an analemma available from Sky Publishing, the publishers of Sky & Telescope.)
Mean time takes all of these variations through the year and smooths them out to produce a system of time usable for clocks with fixed gears or electronic cycles. In fact, it was the development of accurate uniform-rate clocks in the 18th century that led to the use of mean time. This brings us to the problem facing astrologers and the reason for this article.
When?
We don’t know when people switched from apparent time to mean time for daily use or legal record keeping.
The Encyclopaedia Britannica refers to the 17th century introduction of clocks and watches and leaves it at that. The Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac says clocks weren’t accurate enough until the late 18th century. It says “apparent time was gradually superseded during the late eighteenth and early nineteenth centuries by local mean solar time for general civil use.” (p 89).
The following are some fragments of information from Arthur Blackwell (c/o Astrolabe; PO Box 28; Orleans, MA 02653): Geneva switched from LAT to LMT in 1780. London switched in 1792. Paris switched in 1816. In 1834, ephemerides switched from LAT to LMT. An 1838 eclipse was recorded in apparent time in Bridgeton, NJ (about 25 miles from Philadelphia). GMT became legal time in Great Britain in 1848 (with the possible exception of parts of Ireland and Scotland). Charleville, France was using apparent solar time in 1854 when the birth of Rimbaud was recorded. Rural areas generally switched after urban areas.
If all this confusion reminds anyone of time zone and daylight time changes or the messy switch from Julian to Gregorian calendars over several centuries (not to mention the different starting points for years), I’m not surprised. It just shows that old charts can have as many time change problems as modern charts.
Is there someone out there with the urge to take on a lifetime research project and do a good deed for the community?