CELEBRATE ASTRONOMICAL SPRING! On Thursday 20 March 1997, we will mark the passage of the vernal equinox at 1355 Z (or 8:55 AM EST, 7:55 AM CST, and so forth). At this instant the sun will appear to be directly over the equator and it will mark one of the two times during the year when essentially all places on earth experience equal intervals of daylight and equal night during a 24 hour period.
This event is popularly known as the beginning of the astronomical spring season in the Northern Hemisphere, as the length of daylight rapidly increases toward a maximum on the summer solstice early on the morning of 21 June 1997.
Based upon the U.S. Naval Observatory calculations of the times of the start of the seasons (equinoxes, solstices) and the equinox will occur almost 6 hours later this year than it did last year because the earth makes one circuit of the sun once in 365.2422 days. Every fourth year (such as in 1996 and 2000) the insertion of an extra "leap year" day at the end of February essentially corrects for this drift. As discussed in an earlier Supplemental Information file, the Gregorian Calendar provided a better correction scheme than did the Julian Calendar.
other astronomical events until the year 2000, the calculates the times of equinoxes.
until the year 2000.
Subject: FAQ-Equinoxes
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Length of Day and Night at the Equinoxes
Day and night are not exactly of equal length at the time of the March and
September equinoxes. The dates on which day and night are each 12 hours
occur a few days before and after the equinoxes. The specific dates of this
occurrence are different for different latitudes.
On the day of an equinox, the geometric center of the Sun's disk crosses the
equator, and this point is above the horizon for 12 hours everywhere on the
Earth. However, the Sun is not simply a geometric point. Sunrise is defined
as the instant when the leading edge of the Sun's disk becomes visible on
the horizon, whereas sunset is the instant when the trailing edge of the
disk disappears below the horizon. These are the moments of first and last
direct sunlight. At these times the center of the disk is below the horizon.
Furthermore, atmospheric refraction causes the Sun's disk to appear higher
in the sky than it would if the Earth had no atmosphere. Thus, in the
morning the upper edge of the disk is visible for several minutes before the
geometric edge of the disk reaches the horizon. Similarly, in the evening
the upper edge of the disk disappears several minutes after the geometric
disk has passed below the horizon. The times of sunrise and sunset in
almanacs are calculated for the normal atmospheric refraction of 34 minutes
of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at
the tabulated time the geometric center of the Sun is actually 50 minutes of
arc below a regular and unobstructed horizon for an observer on the surface
of the Earth in a level region.
For observers within a couple of degrees of the equator, the period from
sunrise to sunset is always several minutes longer than the night. At higher
latitudes in the northern hemisphere, the date of equal day and night occurs
before the March equinox. Daytime continues to be longer than nighttime
until after the September equinox. In the southern hemisphere, the dates of
equal day and night occur before the September equinox and after the March
equinox.
In the northern hemisphere, at latitude 5 degrees the dates of equal day and
night occur about February 25 and October 15; at latitude 40 degrees they
occur about March 17 and September 26. On the dates of the equinoxes, the
day is about 7 minutes longer than the night at latitudes up to about
25cdegrees, increasing to 10 minutes or more at latitude 50 degrees.
--
Subject: The Dark Days of Winter
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The Dark Days of Winter
The period between the first week in December and the first week in January
could well be called the "dark days" for the mid-northern latitudes. At
latitude 40 degrees north, earliest sunset occurs around 8 December each
year, and latest sunrise occurs around 5 January. The day with the least
amount of daylight is the winter solstice, the first day of winter, around
21 December. Why are not all these dates the same? The answer is not simple.
There are two effects which, together, determine the local time of Sun
phenomena, such as sunrise, sunset, and transit. One is the called the
Equation of Time, the other is the Sun's declination.
The Equation of Time is a way of describing the variation in the time of
Sun-related phenomena within our standard 24-hour timekeeping system. In any
time zone, the Equation of Time is simply the difference between 12:00 noon
on a clock and the actual time of the Sun's transit (sundial noon) across
the central meridian of the time zone. The time between successive transits
of the Sun - the length of the solar day - varies considerably over the
year. It is itself determined by two factors, both dependent on the position
of the Earth in its orbit. Suffice it to say that from mid-November to early
February these two factors work together to make the solar day longer than
24 hours: in late December, as much as 30 seconds longer than 24 hours.
Since we don't adjust our clocks for this effect, the Sun's transit moves
later and later each day during this period.
All other things being equal, the times of all Sun phenomena are tied
directly to the time of transit. But all other things are not equal. The
Sun's declination, its angular distance above or below the equator, changes
on a yearly cycle, causing our seasons. The Sun's declination determines the
maximum height of the Sun in the sky on any given day, hence the azimuth of
the sunrise and sunset points, and the length of time the Sun is above the
horizon. Most of us know the Sun is at its "lowest point in the sky" on the
first day of winter, so we expect the Sun to be above the horizon the least
amount of time that day.
So two effects determine the times of sunrise and sunset: the Equation of
Time and the Sun's declination. But their relative magnitudes vary. In late
December, the daily rate of change of the Sun's declination is quite small
and is, of course, zero at the December solstice; "solstice" means "Sun
stationary". However, the daily rate of change of the Equation of Time
reaches a maximum just a few days later. Thus in late December it is the
Equation of Time that has the dominant influence over the changes in sunrise
and sunset times from one day to the next. In fact, the Equation of Time
dominates, at latitude 40 degrees north, from about 8 December to 5 January.
Outside of these few weeks, the Sun's declination changes are dominant.
These two dates represent the dates on which the magnitudes of the two
effects "cross over" at this latitude. (At higher latitudes, the crossover
dates are closer to the equinox since the declination effect is greater
there.)
The 8 December crossover day is the date of earliest sunset. Why? In the
weeks before solstice, the two effects act in opposite directions on the
time of sunset: the declination effect pulling it earlier and the Equation
of Time pushing it later. On 8 December the Equation of Time begins to
dominate and sunset begins to move later. Meanwhile both effects are pushing
sunrise later and later. After solstice, the situation reverses. Both
effects push sunset later. But for sunrise, the declination effect now pulls
it earlier while the Equation of Time effect continues to push it later. The
Equation of Time prevails until 5 January, when the declination effect takes
over and sunrises begin to move earlier. So 5 January is the date of latest
sunrise.
A similar situation occurs at the summer solstice, although the effect is
not as extreme. Solstice occurs around 21 June, but at latitude 40 degrees
north the earliest sunrise occurs around 14 June and the latest sunset
around 28 June.