SUN for AUGUST 25, 2007, Our Star!
Sun reaches the AUTUMN EQUINOX Sept 23 at 06:51 ADT (Sept 23, 09:51 UT)
 
THE ANALEMMA CHART (the pattern made by the Sun if recorded at the same moment each sunny day for a year), shows the Sun's approximate position (the yellow dot) at Noon for Horton Bluff on the chart date. The orange circled dot indicate the change since just before the Summer Solstice (June 13). This chart is created for 13h 16min 54sec ADT, the moment of Mean Solar Noon at Horton Bluff. The green dots represent Sun's position for each week that preceeds or follows the current Sun position.
 
Earth-Sun related notes:
THE ANALEMMA CHART (the pattern made by the Sun if recorded at the same moment each sunny day for a year) in the upper right shows the Sun's position (yellow dot) at High Noon, about 12:17 AST (12h 16min 54sec) at Horton Bluff, for the date shown. At solar noon at any location, the Sun is above the southern horizon at its highest altitude for the day. I have chosen to develop an analemma weekly, at mean solar noon at the local meridian of Horton Bluff (longitude 64 degrees 13.6 minutes of arc west of the Prime Meridian, at latitude 45 degrees 6.4 minutes of arc north of the equator).
 
One might expect that the Sun would be on the meridian (solar noon) each day at exactly the same time. It would be true if the Sun's apparent daily motion progressed at a uniform rate. Each day it would cross the meridian of Horton Bluff (time of transit) at 12h 16min 54sec AST, however, the motion is irregular and it is this irregularity that is partly responsible for the figure of "8" or "bowling pin" pattern the analemma makes on the chart. This is the pattern made by the Sun if recorded at the same moment each sunny day for a year. The analemma pattern permits one to compare today's position of the Sun (the yellow dot) in relation to a whole year's worth of solar noons.
 
Currently the Sun crosses the Meridian late, making necessary a correction to the sundial (the"slow" time is added to get Mean Solar Noon time; see Sun fast, Sun slow or Equation of Time ).
 
 
A chart with the same relative pattern could be made for any location on Earth. Each chart, however, would show a solar altitude and transit time specific to the location. The grid on this chart shows local altitude above the horizon and the local mean solar time on the meridian. Read on to find out more about this chart. The following link shows the chart drawn on the grid of the Celestial Sphere showing right ascension(RA) and declination (Dec).
 
MERIDIAN and TRANSIT: A rotating Earth makes the Sun appear to rise above the horizon in the east and arch up into into the sky until, near the middle of the day, it reaches its highest altitude in the south. Following this moment, the Sun appears to gradually decend until it slips below the western horizon at sunset, followed by a period of darkness. The pattern, repeating over and over again, establishes the basis for each day on the calendar. Long ago the peak of the high noon Sun was recognized as a unique moment that could be measured and used to reference time of day, seasons and a person's location on the planet's surface.
 
With a stick stuck vertically in the sand and a few days of observation, an observer could deduce that during the middle of the day the stick's shortest shadow gets aligned on the same side of the stick as on previous days. From this shadow observation, one could infer that the Sun at this time was at its highest point in the sky and aligned in the opposite direction. Visualize this as the step that led to the discovery of the concept of a meridian (the word meridian relates to Sun's highest location at "middle day"). Imagine the meridian as a line running from the due south, through the zenith or overhead point , on to the north celestial pole (near the North Star), and down to the horizon at due north. All places on the same meridian have the same longitude. The Sun crossing the meridian each day is referred to as its transit; it marks high noon, the division between the A.M. and P.M. systems of telling time (anti-meridian and post-meridian). The Sun's maximum altitude for the day is reached on the meridian above south at the time of transit (in the northern hemisphere).
 
LONGITUDE: The Prime Meridian (i.e. Longitude 0°, the north-south meridian of Greenwich, England) by international agreement has become the reference meridian for all other meridians around the globe. By comparing the Sun's transit time on the Prime Meridian with the time the Sun crosses a local meridian, time and longitude of that place can be related to all other places. For example: after the Sun transits the meridian of Greenwich, Earth must rotate for an additional period of 4 hours, 16 minutes, 54.4 seconds to bring the Sun across my meridian at Horton Bluff (based on Earth rotating through 1 degree of longitude every 4 minutes). The time interval between the transit of the Sun on the Greenwich meridian and the transit of the Sun on my local meridian establishes the angle of longitude separating the two meridians: longitude 64 degrees 13.6 minutes west of Greenwich. For a similar reason, my time zone, to the nearest hour is 4 hours earlier than Greenwich or Universal Time (9h UT - 4h = 5h AST ).
 
LATITUDE: Measuring the altitude of my local Sun at the moment of transit compares where I am on the globe in relation to the equator (0° latitude on the Earth or declination 0° on the celestial sphere). Sun's declination on the Celestial Sphere tells where the overhead Sun is relative to the equator. How far is it from being overhead for me? Studying the relationship between the Sun's noon altitude above my horizon and the Sun's declination on the Celestial Sphere for a particular day can determine my latitude: my latitude = 90° minus today's noon altitude of the Sun above my horizon + Sun's declination on the Celestial Sphere today. Result: Horton Bluff latitude is close to 45 degrees north of the equator (45° 6'.4 N).
 
Every day, each celestial object crosses the meridian (transits) at a specific time and at some specific altitude. Such crossings are used to relate the positions of celestial objects in the sky, to the locations of places on Earth's surface. Understanding, measuring and applying this relationship is the original key to location, time and navigation on the Earth and in the sky.
 

SUN FAST, SUN SLOW: The apparent motion of the Sun due to Earth's motion is not constant (as would be a Mean Sun, shifting at a uniform rate). This is due to variation in (1) Earth's orbital speed and (2) the daily change in the angle of Earth's tilt relative to the Sun (the inclination of the ecliptic to equitorial plane of Earth's rotation). The result is that from day to day the actual position of the Sun appears to get ahead (fast Sun) or behind (slow Sun) where it would be if the Sun appeared to move at a uniform rate. A Mean Sun would cross the meridian at the same time each day, whereas, the fast Sun crosses the meridian earlier and the slow Sun and crosses the meridian later. This variation gives the shape of an "8" to the analemma, discussed in the next paragraph.

ANALEMMA: The variation in the real Sun's apparent location can be seen if one were to note the position of the Sun on the Earth or in the sky at the same time each day (for example: take a photograph of the Sun from a fixed location at the same time each sunny day). The shape created by a record of the daily change in position resembles a figure of "8" or a bowling pin, as demonstrated in the chart shown to the left or the main chart at the top of the page. The analemma is the name applied to the solar track described. Variation in time produced by this pattern of effects is known as the Equation of Time. Some days the real Sun reaches local solar noon ahead of schedule (the Sun is fast), other days it is behind where it would be (the Sun is slow) if things were running at a uniform rate. These slower or faster times at which the Sun crosses the local meridian can be referred to as "corrections to the sundial" and must be taken into account if a sundial is being used to keep time fairly close to accurate. The analemma was once a common feature placed on world maps and globes, but is rarely seen on maps today. (I have wondered why???).
 
SEASON MARKER: The analemma also gives a concrete, visible, reference to officially mark the beginning of each season. The moment the Sun appears to cross the Celestial Equator as it moves into the Northern Hemisphere marks the moment of theVernal Equinox ( Spring Equinox for folks in the Northern Hemisphere, the Autumn Equinox for folks in the Southern Hemisphere). This usually occurs March 19th or 20th. The Sun's celestial declination, on the celestial sphere, at the moment of the equinox is 0°; it is exactly overhead at some point along Earth's equator (latitude 0°). Declination on the celestial sphere is the equivalent of latitude on the Earth.
 
Summer Solstice: June 21, at 15:06 ADT (June 21, 18:06 UT), the Sun reaches declination 23 degrees 26 min, its highest northern declination. (solstice = sun stands still). At the same time Sun's Right Ascension reaches 06h 00min on the grid of the Celestial Sphere].
 
The most northerly latitude reached by the overhead Sun on the Earth is also its most northerly declination on the celestial sphere. The moment the Sun appears to reach this point, marks the Summer Solstice and beginning of summer in the Northern Hemisphere (the reverse in the Southern Hemisphere). For an instant the Sun stops its daily motion northward and begins its decent southward. The moment to the nearest minute is frequently published for use by the general public. On this day (June 21) the Sun has advanced to its very highest position on the analemma and makes its highest path across the Northern Hemisphere sky, resulting in the longest daylight and the shortest night of the year. Near the solstice, the Sun on the analemma almost "stands still" [solstice from Latin solstitium, from sol sun+ sistere stand still].Show top of the analemma in enlarged detail (June 1 to July 20). The reverse of the process creates the moment to mark the official beginning of autumn and winter.
 
The moment the Sun appears to cross the Celestial Equator as it moves into the Southern Hemisphere marks the moment of the Autumn Equinox for folks in the Northern Hemisphere, the Spring Equinox for folks in the Southern Hemisphere. This usually occurs between September 21st to 23rd. The Sun's celestial declination at the moment of the equinox is 0°, on the celestial sphere; it is exactly overhead at some point along Earth's equator (latitude 0°). Declination on the celestial sphere is the equivalent of latitude on the Earth's sphere. Globally day and night are about equal in duration and sun rises on the east point and sets on the west point. Prior to this in the Northern Hemisphere, day was longer in duration than night; Sun rose and set on the north side of east and west. In the days ahead day is shorter in duration compared to night and Sun rises and sets on the south side of east and west.
 
 
MORE: The north and south shift in declination results from the 23.4 degree angle that Earth's rotating axis is tilted from the plane of its orbit around the Sun. As Earth moves in its orbit, the degree of tilt maintained toward the Sun changes in a predictable repeating annual pattern causing the Sun to shift higher or lower along an apparent path (the ecliptic) that slopes at changing angles against the sky background.
 
Earth orbits the Sun in an elliptical path causing its speed to vary. For example, Earth orbits faster in January when it is closer to the Sun (perihelion). The rules of nature dictate that the planet move faster when closer to the Sun and slower when further from the Sun. In July when we are furthest from the Sun (aphelion), Earth slows down.
 
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