Cycle of the Sun
The daily rising and setting of the sun is familiar to us all and is understood as the result of Earth's rotation about its axis. The sun, however, displays more subtle motions during the year that are produced by a second motion - the orbit of the Earth around the sun.
The Annual Path of the Sun and the Zodiacal Constellations
It is not readily apparent but, during the course of a year the sun glides eastward in the sky by about 1 degree per day. This motion is dwarfed by the daily rising and setting of the sun which is an east to west motion. The gradual eastward motion is called sidereal motion (or "starry motion") while the rising and setting is diurnal or daily motion. Figure 3.1 is an animation showing the eastward drift of the sun along the ecliptic - the name given to the path followed by the sun (the atmosphere has been "turned-off" to enable you to see the stars and constellations that the sun passes through during a year). The ecliptic appears as a thin red line. Above is a thin blue line which marks the celestial equator. The sun, moon and planets are always confined to the same region of the sky - a swath that cuts across the sky and passes through the a group of 12 constellations called the zodiacal constellations. You should also note that the planets and moon are also confined to the same region of the sky - |
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| what does this suggest about the motion of the planets? | Figure 3.1 The eastward drift of the sun with respect to the background stars. The red line that marks the path of the sun is the ecliptic. |
Figures 3.2 and 3.3 show the motion of the sun through the "Houses of the Zodiac". Your "astrological sign" is supposedly marked by the presence of the sun in that constellation. Since there are 12 zodiacal constellations, the sun should spend roughly one month in any given zodiacal house.
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| Figure 3.2 Roll-over image showing the sun in Sagittarius which is one of the "signs" or houses of the zodiac. |
Figure 3.3  The sun "moving" through the houses of the zodiac over the course of one year. |
Example 3.1 If you are a "Scorpio" you birth day should fall between October 23 - November 22. This would suggest that the sun is in the constellation Scorpius during this time. Use Stellarium to test whether this is true or not.
Solution: As a popular television show would conclude - this myth is busted! The sun is in the constellations Virgo and Libra for almost all of the period assigned to "Scorpio". The only way that you can get the zodiacal sign "Scorpio" to correspond to the "correct" location of the sun is to set Stellarium to a date 2000 years in the past. It would appear that astrologers have not taken into account precession in the Earth's rotational axis!Daily and Seasonal Motion of the Sun
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Since the dawn of civilization the sun has been used to mark out the time and the seasons. The sun marks local noon when it crosses the meridian - an imaginary line extending from due south to zenith. Figure 3.4 shows the sun as seen from Edmonton at local noon - 12:40:32 Mountain Standard Time. The sun is at its highest point when it is local noon. Also local noon is not the same as civil noon. Civil time is the time system based on time zones and measured from Greenwich, England.
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| Figure 3.4 The Sun on March 21, 2009 at local noon. |
Example 3.2 Find a city or town in the Prairies for which local noon and civil noon occur at the same time.
Solution: Lumsden, Saskatchewan is one good example. At a geographic latitude of 104.9 degrees W, civil and local noon differ by only a few seconds. Regina would also be a good choice.
Sundials, Analemmas and Subtleties in the Sun's Apparent Motion
Table 3.1 gives the time of local noon in Edmonton for some selected dates in 2020. What is "odd" about these times?
Date (2009) |
Time of Local Noon (MST) |
January 2 |
12:37 |
March 2 |
12:45 |
June 2 |
12:32 |
September 2 |
12:33 |
December 2 |
12:24 |
| Table 3.1 Times of Local noon for Edmonton | |
You might expect local noon to occur at exactly the same time each day for a given location. Table 3.1 shows a surprisingly large variation in the time of local noon. Anyone having a sundial will be acquainted with this phenomenon. Another way to illustrate this is shown below in Figures 3.5a and 3.5b.
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| Figure 3.5a The sun traces out the analemma as it changes its position around the "average" time for local noon during a year | Figure 3.5b The analemma provides a method to "correct" for the varying rate at which the sun moves across the sky. |
Both of these figures illustrate a curious "speeding up" and "slowing down" in the motion of the sun throughout the year. This phenomenon is the combined result of three facts:
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The Earth is tilted at 23.5o relative to the Earth-Sun orbital plane
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The Earth is not only rotating once per day but also revolves around the Sun once every 365.25 days
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The Earth's orbit is not circular but is elliptical. This means that the Earth is moving fastest when nearest the sun (early January) and slowest in early July.
For most Canadians "winter" comes sometime in November. Why then is the first "official" day of winter around December 21? The answer has to do with the fortunate fact that the Earth's rotational axis is tipped at a 23.50angle with respect to the earth-sun orbital plane. The seasons are defined by the position of Earth relative to the Sun and Table 3.2 summarizes this.
| Spring (Vernal Equinox): The sun crosses the Celestial Equator sometime around March 21 and continues to rise above the Celestial Equator. In both spring and fall the earth neither tilts away from or into the sun (depicted by yellow rays in Figures 3.5 - 3.8). For this reason, on the equinox day and night are each 12 hours in duration anywhere on the globe. If you study Figure 3.4 you will also notice that the sun is located at the intersection of the ecliptic and celestial equator on either of the two equinoxes. |  |
| Figure 3.6 | |
| Summer (Summer Solstice): On the Summer Solstice the sun climbs to its highest position in the northern sky which is 23.50 above the celestial equator. During the northern summer the tilt of the northern hemisphere is toward the sun which is why daytime is longer than nighttime during this season. The term solstice refers to the apparent "standing still" of the sun. This means that at local noon around both June 21 and December 21 the altitude of the sun above the horizon changes very little from day-to-day. |  |
| Figure 3.7 | |
| Fall(Autumnal Equinox): The sun crosses the Celestial Equator sometime around September 21 and drops below the Celestial Equator. In both spring and fall the earth's neither tilts away from or into the sun (depicted by yellow rays in Figures 3.5 - 3.8). For this reason, on the equinox day and night are each 12 hours in duration anywhere on the globe. |  |
| Figure 3.8 | |
| Winter (Winter Solstice) in the northern hemisphere occurs when the northern hemisphere tips away from the sun. The first day of winter occurs when the sun has reached its lowest point in the sky -23.50 below the celestial equator. |  |
| Figure 3.9 | |
| Table 3.2 Orientation of Earth relative to the sun at the start of each season. | |
Figure 3.9 summarizes this and helps explain the reason for the seasons. During summer months the earth tilts toward the sun and we receive increased solar heating during this time. During the winter months the opposite is true, the earth tilts away from the sun and we receive less solar energy.
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| Figure 3.10 Orientation of the earth with respect to the sun at the start of each of the four seasons. |
Example 3.3 What is the altitude of the sun on the first day of spring as seen from Saskatoon, Saskatchewan?
Solution: To answer this you will need to find the latitude of Saskatoon. Google Earth (or any other convenient resource) indicates a latitude of 52 degrees North for Saskatoon. As you recall from Chapter 2.2, the altitude of the celestial equator is equal to the compliment of your geographic latitude. In the case of Saskatoon this would mean that the celestial equator has an altitude of 90-52 = 38 degrees. Since the sun is on the celestial equator on either the spring or fall equinox you may conclude that the altitude of the sun will be 38 degrees (at local noon).
The Midnight Sun
There are strange things done in the midnight sun
By the men who moil for gold;
The Arctic trails have their secret tales
That would make your blood run cold;
The Northern Lights have seen queer sights,
But the queerest they ever did see
Was that night on the marge of Lake Lebarge
I cremated Sam McGee.
(The Cremation of Sam McGee, R.W. Service)
Is it really true that there are places where the sun never sets? What is the "midnight sun"? To help answer this and gain some insight into the role that the inclination of the Earth's axis plays in all of this use Stellarium to investigate how the day unfolds in Old Crow, Yukon (67o 34' N ,139o44' W) on a date close to the summer solstice. Beginning on about the 4th of June you will indeed see that the sun does not set - it travels on a shallow arc that just skims the northern horizon.
| The sun continues to do this until about July 8 when it once again begins to set. Any location north of the Arctic Circle (66.5o N) or south of the Antarctic Circle (66.5o S) will experience at least one day for which the sun never sets (or, 6 months later never rises). This is simply due to the fact that the Earth tips toward the sun by 23.5o on the Summer solstice and completely illuminates the polar region. Figure 3.11 shows this. | |
| Figure 3.11 Animation showing the northern polar region completely illuminated on June 21. |
Another Myth "Busted"
A very common misconception is that we have winter because the Earth is farthest from the sun at this time. WRONG! In fact, the closest approach of the earth to the sun occurs in early January! For example, in the year 2020 the closest approach between Earth and Sun occurred on January 5, 2020 (at about 00:48) at when the Earth was 0.983 AU away. The greatest separation will be on July 4, 2020 when the Earth will be 1.0168 AU from the sun.
The earth-sun distance has only a minor affect and certainly does not "cause" the seasons - it's all due to the 23.50 tilt of the Earth's rotational axis.
Motion of the Planets
Even a casual observer of the night sky will have seen, at one time or another, Jupiter, Saturn Venus and Mars. Spotting Mercury takes a bit more persistence - its close proximity to the sun means that you have to know where to look to see it. Figure 3.12 shows a beautiful conjunction of the Moon with Jupiter and Mercury as visible in the early morning sky of September 24, 2003.
Ancient peoples were well acquainted with these five "wandering stars" - or planets. All of these planets share a common property - they are always found very close to the ecliptic - the path followed by the sun across the sky. Figure 3.12 is a roll-over image which shows the ecliptic passing through Mercury and Jupiter.
To better understand the motion of the planets see if you can use Stellarium to reproduce the conjunction shown in Figure 3.12 and then follow the planets over the next few weeks. As you do this you will begin to understand that we and the other planets orbit the sun in roughly the same plane.
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| Figure 3.12 A beautiful conjunction between the Moon, Jupiter and Mercury. |
To understand the annual motion of the sun
Chp2.3,
Chp 4
