Time, Seasons, and Tides

Time, Seasons, and Tides

Celestial Sphere Imagine the sky as a great, hollow, sphere surrounding the Earth. The stars are attached to this sphere--- some bigger and brighter than others--- which rotates around the stationary Earth roughly every 24 hours. Alternatively, you can imagine the stars as holes in the sphere and the light from the heavens beyond the sphere shines through those holes.

Celestial Sphere This imaginary sphere is called the celestial sphere,, and has a very large radius so that no part of the Earth is significantly closer to any given star than any other part. Therefore, the sky always looks like a great sphere centered on your position. The celestial sphere (and, therefore, the stars) appears to move westward--- ---stars rise in the east and set in the west.

Zenith The point straight overhead on the celestial sphere for any observer is called the zenith and is always 90 degrees from the horizon.

Meridian The arc that goes through the north point on the horizon, zenith, and south point on the horizon is called the meridian. Any celestial object crossing the meridian is at its highest altitude during that night or day.

Meridian During daylight, the meridian separates the morning and afternoon positions of the Sun. In the morning the Sun is ``ante meridiem'' (Latin for ``before meridian'') or east of the meridian, abbreviated ``a.m.''. At local noon the Sun is right on the meridian. In the afternoon the Sun is ``post meridiem'' (Latin for ``after meridian'') or west of the meridian, abbreviated ``p.m.''.

Solar Day Every day the Sun rises in an easterly direction, reaches maximum height when it crosses the meridian at local noon, and sets in a westerly direction and it takes the Sun on average 24 hours to go from noon position to noon position the next day. The ``noon position'' is when the Sun is on the meridian on a given day. Our clocks are based on this solar day.

Sidereal Day The fact that our clocks are based on the solar day and the Sun appears to drift eastward with respect to the stars (or lag behind the stars) by about 1 degree per day means that if you look closely at the positions of the stars over a period of several days, you will notice that according to our clocks, the stars rise and set 4 minutes earlier each day. Our clocks say that the day is 24 hours long, so the stars move around the Earth in 23 hours 56 minutes.

Sidereal Day This time period is called the sidereal day because it is measured with respect to the stars. This is the true rotation rate of the Earth and stays the same no matter where the Earth is in its orbit--- ---the sidereal day = 23 hours 56 minutes on every day of the year. One month later (30 days) a given star will rise 2 hours earlier than it did before (30 days 4 minutes/day = 120 minutes). A year later that star will rise at the same time as it did today.

Sidereal Day Another way to look at it is that the Sun has made one full circuit of 360 degrees along the ecliptic in a year of 365.24 days (very close to 1 degree per day). The result is that between two consecutive meridian crossings of the Sun, the Earth has to turn nearly 361 degrees, not 360 degrees, in 24 hours. This makes the length of time for one solar day to be a little more than the true rotation rate of 23 hours 56 minutes with respect to the background stars.

Solar and Sidereal Time as Viewed from Space Imagine that at noon there is a huge arrow that is pointing at the Sun and a star directly in line behind the Sun. The observer is also experiencing local noon. If the Sun were not there, the observer would also see the star on the meridian.

Solar and Sidereal Time as Viewed from Space Now as time goes on, the Earth moves in its orbit and it rotates from west to east (both motions are counterclockwise if viewed from above the north pole). One sidereal period later (23 hours 56 minutes) or one true rotation period later, the arrow is again pointing toward the star. The observer on the Earth sees the star on the meridian. But the arrow is not pointing at the Sun!

Solar and Sidereal Time as Viewed from Space The Earth's sidereal day is always 23 hours 56 minutes long because the number of degrees the Earth spins through in a given amount of time stays constant. The value of 24 hours for the solar day is an average for the year and is what our time-keeping system is based on.

Time Zones People east of you will see the Sun on their meridian (High Noon) before you see it on yours. Those in Denver, Colorado will see the Sun on their meridian about 52 minutes before people in Los Angeles will see the Sun on their meridian. That is because Denver is at longitude 105 West longitude while Los Angeles is at 118 West longitude (or 13 difference).

Time Zones For each one degree difference in longitude a person is from you, the time interval between his local noon and yours will increase by 4 minutes. It used to be that every town's clocks were set according to their local noon and this got very confusing for the railroad system so they got the nation to adopt a more sensible clock scheme called time zones.

Time Zones Each person within a time zone has the same clock time. Each time zone is 15 degrees wide, corresponding to 15 /minute 4 minutes/degree = 60 minutes = 1 hour worth of time. Those in the next time zone east of you have clocks that are 1 hour ahead of yours.

Equation of Time There is a further complication in that the actual Sun's drift against the stars is not uniform. Part of the non-uniformity is due to the fact that on top of the general eastward drift among the stars, the Sun is moving along the ecliptic northward or southward with respect to the celestial equator. Thus, during some periods the Sun appears to move eastward faster than during others. Apparent solar time is based on the component of the Sun's motion parallel to the celestial equator.

Equation of Time This effect alone would account for as much as 9 minutes difference between the actual Sun and a fictional mean Sun moving uniformly along the celestial equator.

Equation of Time Another effect to consider is that the Earth's orbit is elliptical so when the Earth is at its closest point to the Sun (at perihelion), it moves quickest. When at its farthest point from the Sun (at aphelion), the Earth moves slowest.

Equation of Time At perihelion the Earth is moving rapidly so the Sun appears to move quicker eastward than at other times of the year. The Earth has to rotate through a greater angle to get the Sun back to local noon. This effect alone accounts for up to 10 minutes difference between the actual Sun and the mean Sun.

Equation of Time Rather than resetting our clocks every day to this variable Sun, our clocks are based on a uniformly moving Sun (the mean Sun) that moves along the celestial equator at a rate of 360 degrees/365.2564 per day.

Seasons The seasonal temperature depends on the amount of heat received from the Sun in a given time. To hold the temperature constant, there must be a balance between the amount of heat gained and the amount radiated to space. If more heat is received than is lost, your location gets warmer; if more heat is lost gained, your location gets cooler.

Seasons What causes the amount of energy reaching a given location during the day to change throughout the year?

Seasons Three popular theories are often stated to explain the temperature differences of the seasons: 1) the different distances the Earth is from the Sun in its elliptical orbit (at perihelion the Earth is 147.1 million kilometers from the Sun and at aphelion the Earth is 152.1 million kilometers from the Sun); and 2) the tilt of the Earth's axis with respect to its orbital plane. 3) A popular variation of the distance theory says that the part of the Earth tilted toward the Sun should be hotter than the part tilted away from the Sun because of the differences in distances.

First Theory for Seasons If the first theory were true, then both the north and south hemispheres should experience the same seasons at the same time. They do not.

Third Theory for Seasons The 23.5 tilt of the Earth means that the north pole is about 5080 kilometers closer than the south pole toward the end of June. This is much, much smaller than the 152 million kilometer distance between the Sun and the Earth's center at that time. The amount of energy received decreases with the square of the distance.

Third Theory for Seasons If you calculate (152,000,000 + 5080) 2 /(152,000,000-5080) 2, you will find that the north pole would get slightly over 1/100th of one percent more energy than the south pole. This is much too small a difference to explain the large temperature differences! Even if you compare one side of the Earth with the opposite side, so you use the Earth's diameter in place of the 5080 kilometers in the calculation above, you get 3/100th of one percent difference in energy received. Clearly, distance is not the reason for the large temperature differences.

Second Theory for Seasons The tilt theory correctly explains the seasons but the reason is a little more subtle than the distance theory's explanation. Because the Earth's rotation axis is tilted, the north hemisphere will be pointed toward the Sun and will experience summer while the south hemisphere will be pointed away from the Sun and will experience winter.

Second Theory for Seasons During the summer the sunlight strikes the ground more directly (closer to perpendicular), concentrating the Sun's energy. This concentrated energy is able to heat the surface more quickly than during the winter time when the Sun's rays hit the ground at more glancing angles, spreading out the energy.

Second Theory for Seasons

Second Theory for Seasons Also, during the summer the Sun is above the horizon for a longer time so its energy has more time to heat things up than during the winter.

Reason for the Season The apparent yearly path of the Sun through the stars is called the ecliptic. This circular path is tilted 23.5 degrees with respect to the celestial equator because the Earth's rotation axis is tilted by 23.5 degrees with respect to its orbital plane.

Reason for the Season The ecliptic and celestial equator intersect at two points: the vernal (spring) equinox and autumnal (fall) equinox. The Sun crosses the celestial equator moving northward at the vernal equinox around March 21 and crosses the celestial equator moving southward at the autumnal equinox around September 22.

Reason for the Season When the Sun is on the celestial equator at the equinoxes, everybody on the Earth experiences 12 hours of daylight and 12 hours of night for those two days (hence, the name ``equinox'' for ``equal night''). The day of the vernal equinox marks the beginning of the three-month season of spring on our calendar and the day of the autumn equinox marks the beginning of the season of autumn (fall) on our calendar. On those two days of the year, the Sun will rise in the exact east direction, follow an arc right along the celestial equator and set in the exact west direction.

Reason for the Season When the Sun is above the celestial equator during the seasons of spring and summer, you will have more than 12 hours of daylight. The Sun will rise in the northeast, follow a long, high arc north of the celestial equator, and set in the northwest. Where exactly it rises or sets and how long the Sun is above the horizon depends on the day of the year and the latitude of the observer.

Reason for the Season When the Sun is below the celestial equator during the seasons of autumn and winter, you will have less than 12 hours of daylight. The Sun will rise in the southeast, follow a short, low arc south of the celestial equator, and set in the southwest. The exact path it follows depends on the date and the observer's latitude.

Reason for the Season

Tides The ocean tides are caused by different strengths of the Moon's gravity at different points on the Earth. The side of the Earth facing the Moon is about 6400 kilometers closer to the Moon than the center of the Earth is, and the Moon's gravity pulls on the near side of the Earth more strongly than on the Earth's center.

Tides on the Near Side This produces a tidal bulge on the side of the Earth facing the Moon. The Earth rock is not perfectly rigid; the side facing the Moon responds by rising toward the Moon by a few centimeters on the near side. The more fluid seawater responds by flowing into a bulge on the side of the Earth facing the Moon. That bulge is the high tide.

Tides on the Far Side At the same time the Moon exerts an attractive force on the Earth's center that is stronger than that exerted on the side away from the Moon. The Moon pulls the Earth away from the oceans on the far side, which flow into a bulge on the far side, producing a second high tide on the far side.

Affect of Moon s Gravity

Tides These tidal bulges are always along the Earth- Moon line and the Earth rotates beneath the tidal bulge. When the part of the Earth where you are located sweeps under the bulges, you experience a high tide; when it passes under one of the depressions, you experience a low tide. An ideal coast should experience the rise and fall of the tides twice a day. In reality, the tidal cycle also depends on the latitude of the site, the shape of the shore, winds, etc.

Higher and Lower Tides The Sun's gravity also produces tides that are about half as strong as the Moon's and produces its own pair of tidal bulges. They combine with the lunar tides. At new and full moon, the Sun and Moon produce tidal bulges that add together to produce extreme tides.

Spring Tides These are called spring tides (the waters really spring up!).

Neap Tides When the Moon and Sun are at right angles to each other (1st & 3rd quarter), the solar tides reduce the lunar tides and you have neap tides

Tides Slow Earth Rotation As the Earth rotates beneath the tidal bulges, it attempts to drag the bulges along with it. A large amount of friction is produced which slows down the Earth's spin.

Tides Slow Earth Rotation The day has been getting longer and longer by about 0.0016 seconds each century. Astronomers trying to compare ancient solar eclipse records with their predictions found that they were off by a significant amount. But when they took the slowing down of the Earth's rotation into account, their predictions agreed with the solar eclipse records. Growth rings in ancient corals about 400 hundred million years old show that the day was only 22 hours long so that there were over 400 days in a year. In July 1996 a research study reported evidence, from several sedimentary rock records providing an indicator of tidal periods, that the day was only 18 hours long 900 million years ago.

Tides Enlarge Moon Orbit Friction with the ocean beds drags the tidal bulges eastward out of a direct Earth-Moon line and since these bulges contain a lot of mass, their gravity pulls the moon forward in its orbit. The increase in speed enlarges the Moon's orbit.

Tides Enlarge Moon Orbit Currently, the Moon's distance from the Earth is increasing by about 3 centimeters per year. Astronomers have been able to measure this slow spiraling out of the Moon by bouncing laser beams off reflectors left by the Apollo astronauts on the lunar surface. The slow spiraling out of the Moon means that there will come a time in the future when the angular size of the Moon will be smaller than the Sun's and we will not have any more total solar eclipses!

Tides Enlarge Moon Orbit Fifty billion years in the future the Earth day will equal 47 of our current days and the Moon will take 47 of our current days to orbit the Earth. Both will be locked with only one side facing the other. People on one side of the Earth will always see the Moon while people on the other side will only have legends about the Moon that left their pleasant sky.