Suns annual motion and the Seasons Living on

Sun’s annual motion and the Seasons Living on Earth may be expensive, but it includes a free yearly trip around the Sun.

Review Question From a latitude of 40 degrees north, describe where in the sky you would find the zenith, the meridian and the north celestial pole.

Review Question From 40 degrees north, describe where in the sky you would find the south celestial pole?

Review Question From 40 degrees north, describe where in the sky you would find the celestial equator?

Review Question From a latitude of 40 degrees south, describe where in the sky you would find the zenith, the meridian and the south celestial pole and the celestial equator.

Review Question Describe the diurnal motion of the Sun.

Discussion About what time of day is depicting in this drawing? How can you tell?

Discussion About where in the sky will the Sun be at 6 pm?

Review Question Describe how the stars move through the sky if you were at the earth’s south pole.

Star trails…

Review Question Describe how the stars move through the sky if you were at the earth’s equator.

Star trails…

Review Question Which star (A, B, C, or D) will spend the least amount of time above the horizon during the course of a day?

Review Question As viewed from the northern hemisphere, which stars spend a greater amount of time above the horizon, those north of the celestial equator or those south of the celestial equator?

Star trails over the Gemini Northern Telescope dome. The glow from the eruption of the Kilauea volcano is seen on the left. Is the camera facing north or south?

Review Question What is the definition of a solar day? What is the definition of a sidereal day?

Discussion If I told you the camera was pointed east, which way are the stars moving?

Discussion If I told you the camera was pointed east, are you north or south of the equator?

Discussion How can you use the picture to determine your latitude?

Discussion In what constellation will the Sun be in at sunset?

Review Question What time of year is the solar day the longest? What time of year is the sidereal day the longest?

Review Question If the Earth moves faster in its orbit what happens to the length of the sidereal day?

Review Question If the Earth moves faster in its orbit what happens to the length of the solar day?

Review Question In what constellation will the Sun be in one month from the time depicted in this drawing?

Review Question What is the ecliptic?

The Seasons and tilt The seasons are caused by the 23. 5 degree tilt of the Earth’s rotation axis relative to the normal (perpendicular) of Earth’s orbital plane.

Conservation of angular momentum Anything that spins on an axis or revolves around another object has angular momentum. Conservation of angular momentum requires that the rate of spinning remains constant with time. Also, the axis of rotation of any spinning object remains in a fixed direction in space.

Discussion How does conservation of angular momentum help you when you are riding a bicycle?

Near edge-on view

Discussion Where is the Sun today on the previous picture?

Discussion What is the evidence that the seasons are not caused by the Earth’s distance from the Sun?

The seasons and ellipticity The Earth’s orbit is nearly circular – distance from the Sun varies by only 3% Earth is closest to the Sun in January and furthest from the Sun in July Summer in northern hemisphere is winter in southern hemisphere

Discussion What are the two reasons it is warmer during the summer than during the winter?

Why is summer warmer? 1. The Sun, being above the celestial equator, remains in the sky longer during the summer, the longest daylight time occurring on the summer solstice. 2. The Sun rays hit the earth more directly during the summer months, i. e. the summer hemisphere receives more energy per square meter.

Discussion If I make the Earth’s orbit a perfect circle, do the seasons in Cleveland stay the same, become more extreme or become less extreme?

Discussion From Cleveland, latitude 42, on what day does the Sun pass through the zenith?

Discussion From Cleveland on September 22 the Sun sets directly in the west. Two weeks later, about where would we see the set?

The arctic circle is the northern latitude at which on the summer solstice the Sun never sets and on the winter solstice the Sun never rises above the horizon.

Discussion What is the latitude of the arctic circle?

Discussion The tropics mark the farthest points north and south where the Sun can appear at the zenith. What is the latitude of the tropics?

Discussion What is the highest the Sun can get above the southern horizon at a latitude of 42 degrees?

A complication The Earth’s rotation axis is not really fixed.

The Earth’s rotation rate is nearly constant, its speed is not The speed at which points on the Earth’s equator are moving is larger than points on the Earth at higher latitudes. At the equator you would be moving at 1, 650 km/hr, while at the north pole you would not be moving at all, just rotating around a point.

Discussion Why is Kennedy Space Center in Florida and not in Maine?

Florida is closer to the equator and a space shuttle just sitting on the launch pad is moving about 1, 550 km/hr. If we moved the launch pad to Maine, the space shuttle sitting on the launch pad is only moving 1, 275 km/hr or about 275 km/hr less than in Florida. To launch the space shuttle in Maine would require more fuel to accelerate the shuttle the extra 275 km/hr.

Equatorial Bulge The extra velocity at the equator pushes the matter out at the equator. The same thing happens on a merry-go-round. The closer you are to the edge, the faster you will be moving, and the greater the force pushing you off.

The Earth is not a perfect sphere The Earth bulges out at its equator, that is, its diameter measured along its equator is 43 km larger than its diameter from the north to the south pole.

Oblateness

Precession

The Sun and Moon apply a torque to the Earth Because the Earth is not a perfect sphere and its rotation axis is tilted 23. 5 degrees from its orbital plane, the Sun and the Moon pull on the extra mass in Earth’s equatorial bulge and try to straighten out the tilt.

Precession This off axis force, or torque on the Earth causes Earth’s rotation axis to vary slightly, or precess, over long periods of time. Thus, the celestial poles trace out a circle against the stars over a period 26, 000 years.

Discussion The Great Pyramid at Giza has a tunnel which points toward the north celestial pole. At the time the pyramid was built, around 2600 BCE, toward which star did it point?

Precession of the Equinoxes If the celestial poles change with time, so to must the celestial equator, which is 90 degrees away from the poles. If the celestial equator changes with time, than the intersection of ecliptic and the celestial equator will also change with time.

Age of Aquarius? The “age” is defined by the constellation the Sun is in on the vernal equinox. Currently, it is the age of Pisces.

Discussion Why is the changing of the equinox positions important?

Tropical year The length of our calendar year is defined by the time it takes between two vernal equinoxes. This is called the tropical year and is 365 days 5 hours 48 minutes and 46 seconds long.

Sidereal year However, because the equinox positions slowly changes, this is not the Earth’s actual orbital period around the Sun. The actual orbital period is called the sidereal year and is 365 days, 6 hours, 9 minutes and 10 seconds, or 20 minutes 24 seconds longer.

What is the difference? If we used the sidereal year for our calendar year the seasons would slowly migrate through the different months over time. Spring would come one day earlier every 70 years.

Leap Years There are 356. 242 days per tropical year, or about a quarter of day more than 365. To make everything come out even we therefore add an extra day to our year every four years.

Astronomical coordinates Because of precession and the changing position of the celestial poles and the vernal equinox, celestial coordinates of astronomical objects change from year to year.

Astrology Precession breaks the cause and effect relationship between the position of the stars and seasonal changes.