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Chapter 3
Diurnal Motion, Time and the Calendar
3-A. Diurnal Circles
A casual inspection of the sky reveals that all celestial objects rise on the eastern horizon, cross the local celestial meridian, where an object’s altitude is a maximum, and set on the western horizon. This is an apparent motion caused by the rotation of the Earth around its axis and is referred to as the diurnal rotation of the celestial sphere. The angular rate of rotation of the Earth is 15 degrees per hour, since the Earth rotates through 360 degrees in 24 hours. Hence, celestial objects appear to move in the sky along paths at this rate, but in the opposite direction, that is, from east to west. The path that an object follows across the sky from east to west sky because of this motion is called its Diurnal Circle.
Diurnal circles are the apparent paths that objects appear to follow in the sky as a result of the rotation of the Earth around its axis.
Now the right ascension and declination of an object are not affected by the diurnal rotation of the celestial sphere, but altitude and azimuth are. Since the sky appears to turn around an axis through the celestial poles and in a direction parallel to the celestial equator , a diurnal circle is actually the same as the object's parallel of declination. Thus all objects with the same declination trace out the same diurnal circle. A few of these circles are shown in Figure 3-1.
Figure 3-1. Diurnal circles drawn on the celestial sphere for several different declinations, as seen from latitude 40o^ N.
Notice that diurnal circles make an angle with the horizon but they are all parallel to one another. From the geometry of the situation (see Fig. 3-1), it can be shown that the celestial equator (CE) intersects an observer’s celestial horizon at the east and west points. Thus the diurnal circle that passes through the east point of the horizon and west point of the horizon is the celestial equator (CE). Recall that every point on the celestial equator is 90o^ from either of the two celestial poles. Therefore, the angle that the celestial equator makes with the horizon is always equal to 90o minus the observer's latitude (90o^ - ). The time that an object spends above the horizon depends essentially on where it is located with respect to the celestial equator, i.e., its declination. Half of the equator is above the horizon and so any object on it (declination 0o) is above the horizon for 12 hours and below the horizon for 12 hours. Such an object rises at the east point and sets at the west point. Those objects with declinations between 0o^ and +90o, rise in the northeast, remain above the horizon for more than twelve hours and set in the northwest. Those objects with declinations between 0o^ and -90o, rise in the southeast, are above the horizon for less than twelve hours and set in the southwest.
Now the horizon system of circles is fixed with respect to an observer. Therefore, all the reference circles of this system do not share in the rotation of the celestial sphere. This means the altitude and azimuth of an object on the celestial sphere both change with time and location, but remember, the right ascension and declination do not. Hence, the horizon system is only useful for locating objects on the celestial sphere at a given location on the Earth and at a given instant of time. So, as the celestial sphere appears to turn around its axis, objects in the sky move relative to the horizon and the local celestial meridian (LCM). As we saw in the previous chapter, the axis of rotation lies in the plane of the local celestial meridian and it is tilted relative to the celestial horizon by an amount equal to the observer's geographic latitude. When an object crosses the upper local celestial meridian, it is said to be at upper transit (UT). At upper transit the object is at its greatest distance above the horizon, that is, its altitude is a maximum. When the object crosses the lower local celestial meridian, it is said to be at lower transit (LT). At LT , an object is at its minimum angular distance (in the algebraic sense) from the horizon. When an object is below the horizon, it's angular distance from the horizon, or altitude, is assigned a negative value. Objects make one upper transit and one lower transit each day. For an observer in the Northern Hemisphere, both upper and lower transit will occur above the horizon for stars located near the north celestial pole. That is, for objects with high declinations. Such objects will have their diurnal circles entirely above the celestial horizon and, therefore, can be seen every night. Such objects are called circumpolar stars of perpetual apparition (stars that are always visible), since they circle around the pole star. By symmetry, for this same observer, there will be a range of declinations for stars near the south celestial pole that never rise above the horizon. That is, their diurnal circles are completely below the horizon and such an object can never be seen at that latitude. These stars that never rise are called circumpolar stars of perpetual occultation.
Which objects are circumpolar and which are not depends on the observer's latitude and the declination of the object.
Objects located on the celestial equator rise exactly at the east point, while objects north or south of the celestial equator rise to the north or south of the east point. It is very important to realize that objects, which rise exactly at the east point, must set exactly at the west point. Likewise, objects that rise in the northeast set in the northwest and objects that rise in the southeast set in the southwest.
As discussed previously, the time that an object spends above the horizon depends essentially on where it is located with respect to the celestial equator, i.e., its declination. Half of the equator is above the horizon and so any object on it (declination 0o) is above the horizon for 12 hours and below the horizon for 12 hours. Such an object rises at the east point and sets at the west point.
3-B. Hour Angle
Another important angle related to diurnal motion and time is an object’s hour angle.
Hour Angle (HA) is the angular distance of an object measured westward from the Local Celestial Meridian.
It is sometimes convenient to use positive and negative numbers to describe the position of an object with respect to the Local Celestial Meridian. Some authors refer to such an angle as the meridian angle, t.
When the object is west of the Meridian, t is denoted as positive and when it is east of the Meridian t is negative.
But, it is often convenient to use the hour angles that are negative or positive also, contrary to the above definition.
The hour angle or meridian angle changes from negative to positive when the object crosses the Upper Local Celestial Meridian, moving east to west. As was noted above, when an object crosses the upper LCM, its altitude is a maximum and the object is said to be at Upper Transit (UT). The hour angle changes sign again when the object crosses the lower Local Celestial Meridian twelve hours later. At this time, the altitude of the object is a minimum and it said to be at Lower Transit (LT).
celestial equator by 23.5o. This angle is referred to as the obliquity of the ecliptic. Therefore, as the Sun appears to move around the ecliptic over the course of a year, its angular distance from the celestial equator (declination) changes. The 2 points of intersection of the ecliptic and the celestial equator are called the equinoxes. Now, the Earth takes 365.2564 days to complete a 360o^ orbit around the Sun. Therefore:
In the sky, the Sun appears to move eastward along the ecliptic at the rate of about one degree per day, relative to the "fixed stars."
This apparent motion of the Sun along the ecliptic is merely a reflection of the Earth’s revolution. On or about March 21, the Sun appears to cross the celestial equator moving from south to north. The Sun is then at the point labeled V in Figure 3-3, which is called the vernal equinox. The Sun reaches its maximum northerly declination (+23.5o) on or about June 21. It is then at the point labeled S, which is called the summer solstice. The Sun then crosses the celestial equator, moving from north to south on or about September 22 and is at the point labeled A, called the autumnal equinox. The point where the Sun reaches its maximum southerly declination (-23.5o) is the winter solstice, W.
Hence, twice a year the Sun rises exactly at the East Point of the horizon and sets exactly at the West Point. These are the days when the Sun is at the equinoxes. On these two days, the Sun is above the horizon for twelve hours and below the horizon for twelve hours. See the diagram below, which depicts the diurnal circles of the Sun as seen from Los Angeles. So, near mid-latitude in the summer, the Sun rises in the northeast and sets in the northwest, with the daylight portion of the day longer than the nighttime portion. The day of the year with the longest daylight period occurs about June 21, when the Sun is at the Summer Solstice. During the winter, the Sun rises in the southeast and sets in the southwest, with the amount of daylight hours is less than the amount of nighttime hours. The day of the year
with the shortest amount of daylight occurs about December 22, when the Sun is at the Winter Solstice. All of these things are related to the fact that the diurnal circle of the Sun slowly changes from one day to the next, which means the declination of the Sun changes slowly throughout the year.
The equinoxes and solstices are associated with the above dates only in the sense that the Sun is located at these points on or about those dates. Actually these dates may change by a few days from one year to another. The equinoxes and solstices should be thought of as positions on the celestial sphere rather than as specific dates. For example, the vernal equinox is a point on the celestial sphere located in the constellation of stars called Pisces. Like other points on the celestial sphere, an observer on the Earth sees the solstices and equinoxes to execute diurnal circles that depend on the declination of these points. The vernal equinox rises at the east point of the horizon and sets at the west point of the horizon every day throughout the year. Hence, when the Sun is at the vernal equinox, it also rises at the east point and sets at the west point.
The following table lists the declinations and right ascensions for some important points in the sky. A " U " means undefined. Note that objects on the celestial equator have declinations of zero degrees and that objects south of the
celestial equator have negative declinations. The right ascension and declination of the Sun's position vary as it moves along the ecliptic from one point to another as shown in the above table. The time required for the Sun to cycle through its coordinates, that is, the time it takes the Sun to go once around the ecliptic from the vernal equinox back to the vernal equinox is called the year of the seasons or tropical year and has a length of 365.2422 days. The time it takes for the Earth to revolve 360 degrees in its orbit around the Sun is called the Sidereal Year and has length of 365.2564 days.
A TABLE OF SOME DECLINATIONS AND RIGHT ASCENSIONS
The latter is longer than the tropical year by about 20 minutes because of the precession of the Earth’s axis of rotation (See section 3-I and Chapter 4 on precession). The Moon cycles through its coordinates in one sidereal month or 27. days. The time required for a planet to do this depends on its period of revolution. Because the orbits of the planets of our Solar System lie nearly in the same plane as the Earth’s orbit, the planetary orbits are circles in the sky close to the ecliptic. The belt of the sky, centered on the ecliptic and where most of the planets are observed, is called the zodiac. The ancients organized the stars that are located in this belt into twelve constellations known as the zodiacal constellations. The Sun moves from one zodiacal constellation into the next one in about one month. Visualize this in Figure 3-3.
3-D. Time by the Sun
Time is the interval between two measurable events, such as the ticks of a clock or the beat of a heart. Before the invention of clocks, people often used the apparent motion of the Sun across the sky to tell time. Time using the visible, apparent, or real Sun is called local apparent solar time and abbreviated as LAT. Local apparent solar time is defined to be the hour angle of the Sun ( HA ) plus twelve hours.
LAT = HA + 12:
This is the time indicated on a sundial, that is, it is time by the position of the Sun in the sky relative to one’s local celestial meridian. The event taking place here is the upper transit of the Sun. A long time ago it was decided that this interval should be called a day and that it consisted of exactly 24 hours. However, LAT is not the time we use today on our clocks. There are two reasons for this. First, the real Sun moves at a varying rate along the ecliptic as a result of the Earth's variable speed in its elliptical orbit around the Sun. This means that the interval between upper transits of the Sun changes throughout the year. Secondly, the obliquity of the ecliptic also causes the length of the apparent solar day to be different at different times of the year. Hence, keeping time by the real Sun is not practical. The time we use on our clocks is called zone time, ZT. Everyone living within a specified zone of longitudes agrees to use the same time on their clocks, regardless of the location of the Sun in the sky. This is necessary in a world where people travel relatively large distances very quickly. Zone time uses what is called the local mean solar time, LMT , of the central meridian of a time zone. Everyone has a unique LMT that depends on their longitude. LMT is time kept by the position in the sky of a fictitious object called the Mean (or average) Sun that moves at a uniform rate around the celestial equator instead of the ecliptic.
LMT is the hour angle of the Mean Sun, as seen from a given longitude, plus 12 hours
Now we can define zonetime:
ZT is the hour angle of the Mean Sun, as seen from the central meridian of a time zone, plus 12 hours.
POINT DECLINATION
(Degrees)
RIGHT ASCENSION
(Hours)
VE 0.00 0.
SS +23.5 6.
AE 0.00 12.
WS - 23.5 18.
NCP +90.00 U
SCP - 90.00 U
LST is 12:00 at noon on the day of the autumnal equinox. Thereafter, the sidereal clock gains approximately 4 (3m^56 s) minutes per day on the solar clock.
Sample Problem:
At what EST will M1, the Crab Nebulae, make upper transit at TCNJ on September 15?
Solution:
First we need to find the RA for M1. This may found via the internet or a star chart. The answer is 5h^ 34.5m. Then we need to find the number of days between Sept. 23 (this happens to be the date of the autumnal equinox in
- and Sept. 15. The answer is 8. So 8 days x 4 min/day = 32 min or 31.5 minutes to be more exact. But the sidereal clock in this case is going to be behind the apparent solar clock by this amount and will catch up to the solar clock on Sept. 23rd. Now when M1 is at UT, the LST = RA of M1 = 5:34.5. On Sept. 15, the solar clock is ahead of the sidereal clock by 31.5 minutes. So the LAT = 5:34.5 + 0:31.5 = 6:06.0. Now we must apply the equation of time to find the LMT. From the chart above this is about +4m. Hence, LMT = LAT – ET = 6:06 – 0:04 = 6:02. (we are using a 24 hour clock here, so this in the morning) Corrections must then be made for longitude and the fact that we may be using
daylight saving time.
3-F. Spherical Trigonometry
The distance between any two points in the sky is an angle that is represented by an arc between these 2 points on the celestial sphere. See Chapter 1. This arc is always a section of a great circle. Very often in astronomy we would like to compute such a distance by setting up a spherical triangle in the sky that connects 3 points. Consider a general spherical triangle on the celestial sphere that connects any three points A, B, and C, as depicted in the next figure. The sides of the triangle are a, b, and c. Though they are sides, they are also angles. Such a triangle also includes 3 angles, such as, BAC. We shall refer to this angle simply as A. The other two included angles are B and C. Without proof, we give here the three most important relations for a spherical triangle. The proofs may be found in Smart (1965) or Binnendijk (1960). For the general triangle shown, the Law of Sines is:
sin a/Sin A = sin b/Sin B = sin c/SinC (1)
The Law of Cosines is:
cos a = cos b cos c + sin b sin c cos A. (2)
The Relation Between Two Angles and Three Sides is:
sin a cos B = sin c cos b – cos c sin b cos A. (3)
Of course, permutations of the sides and included angles in the last two
relations are valid.
3-G. Solving the Astronomical Triangle
An example of solving such triangle is done when wants to know what time an object will rise, set, or have any hour angle. For example, the angle t in the next figure is the meridian or hour angle of an object ( t is the included angle ZPS ). The side z is the zenith distance or complement of the altitude, 90o^ – h, and the arc between the pole, P, and
some object, S, is 90o^ - . As in plane trigonometry, if we know any three parts of the triangle, we can solve for any of the other parts.
Let us say we want to find out the hour angle of the Sun when it sets. Solving the triangle for t will enable us to calculate the apparent solar time when the Sun sets, since
LAT = HA + 12:00. (4)
To do this we need to know the declination, , of the Sun on a given day and the observer’s latitude, ф. Hence, the arc (90 – ф) is assumed known. So, using the proper equation, we can solve for t. This may be done using equation (2) with A = t, a = 90o^ - h, b = 90o^ – ф and c =90o^ – . Substituting these parameters into (2) we obtain:
cos (90o^ – h) = sin h = sin sin + cos cos cos t. (5)
Here we have used the relations cos (90o-x) = sin x, and sin (90o-x) = cos x. Now solve (5) for cos t and we have:
cos t = [sin h - sin sin cos cos (6)
Nominally, we want h to be zero, so (6) simplifies to:
cos t = [- sin sin cos cos
cos t = - tan tan
Hence, the meridian angle of an object when it sets depends only on the observer’s latitude and the declination of the object. When evaluating t, one must be wary of the quadrant of the angle. When using (4) to compute the time of sunset, remember to apply the equation of time when finding local mean time or civil time and then correct for longitude to obtain zone time. In the above derivation, one assumes the object is a point source. For stars this is valid but for extended objects such as planets and the Sun, this is not valid and one must decide what is meant by rising and setting. For example, for sunset one must decide what point on the Sun is on the horizon. This is usually taken as the last point on the upper limb of the Sun to be visible.
Assignment 3-G 1:
Find the zone time the center of the Sun sets at latitude 40o^ N. on the day of the winter solstice. The value of the equation of time may found from the graph presented above in section D of this chapter. Then, we may write a program that does this for any values of the input variables.
For a more precise determination of sunset, one should also allow for refraction in the atmosphere. The amount of refraction is a rather uncertain amount because it depends on the temperature and density of the Earth’s atmosphere and these are variables.
Do problem 3-G.
3-H. The Anomalistic Year
The major axis of the Earth’s elliptical orbit around the Sun is also known as the line of apsides. The line of apsides connects the perihelion and aphelion points of the Earth’s orbit. Gravitational perturbations of the Earth’s orbital motion are caused by the other planets. One of the results is to cause the line of apsides to rotate in the plane of the orbit eastward, that is, counterclockwise when viewed from the north ecliptic pole. The time for the Earth to revolve in its orbit from perihelion to perihelion is called the anomalistic year. The anomalistic year has a length of 365.25964 days.
Assignment 3-C.1:
Print this page and submit for grading. For latitude 40o^ N., find the NCP and SCP, draw the axis of rotation, the celestial equator, and the diurnal circle of the Sun on the days of the solstices and the equinoxes. Label everything you draw. See the instructions for drawing diurnal circles in the Appendices.
