Earth Movement and Earth Movement and Solar Calendar Solar - - PowerPoint PPT Presentation

Earth Movement and Earth Movement and Solar Calendar Solar Calendar

Recitation 2 Recitation 2

Elliptical Orbits Elliptical Orbits

The planets move along elliptical orbits around the sun. This results in both a variation in the distance from the sun, and variation in the rotational velocity of each planet.

The earth is closest to the sun at the perihelion, furthest from the sun at the aphelion (the eccentricity of the orbit is exaggerated above for easy visualization)

Tilt Tilt

The earth’s rotational axis is tilted from normal (right angle) to its orbital plane by roughly 23.5º. The tilt causes direct sunlight to fall on different parts of the earth of the course of a year.

Sunlight on tilted Earth Sunlight on tilted Earth

Tilt is responsible for the seasons. As the tilted earth moves around the sun, direct sunlight moves north from the equator at the vernal equinox, to 23.5º north latitude at the summer solstice, back to the equator at the autumnal equinox, and south 23.5º south latitude at the winter solstice.

Sunlight on tilted Earth Sunlight on tilted Earth

Meridians – great circles Longitude – arc angle from prime

Polar Coordinates Polar Coordinates

Parallels – small circles & equator Latitude – arc angle from equator

Important Latitudes Important Latitudes

Please note: 23.5o is the same as 23o 30’

Apparent Solar Day Apparent Solar Day

One rotation = 86,164 s Mean solar day = 86,400 s

The length of a day depends on what you’re watching. The time between seeing the sun appear at its highest point in the sky is longer than the time between seeing a star appear at its highest point in the sky. The sun is much closer than any other star, as such the earth has to rotate a little further to bring it back to the same place in the sky.

Orbit Effect Orbit Effect

The elliptical nature of the earth’s orbit means that the time between sightings

• f the sun overhead is shorter in early April and later in October. If their were

no tilt to the earth, the sun would always shine directly on the equator, and the distribution of times between the sun passing its highest point in the sky would look like the graph above.

Tilt Effect Tilt Effect

Tilt is also affects the time between sightings of the sun at local noon. If the earth moved around the sun in a circular orbit (not elliptical), the time between the sun’s appearance at its highest point in the sky would look like this graph.

Orbit and Tilt Variation

• 20
• 15
• 10
• 5

5 10 15 20 21- Dec 20- Jan 19- Feb 21- Mar 20- Apr 20- May 19- Jun 19- Jul 18- Aug 17- Sep 17- Oct 16- Nov 16- Dec Late (Minutes) Early

However, earth is rotating at a tilt and moving along and elliptical orbit. The net effect of both motions produces the above distribution of times between local noon events.

Photographing the sun or marking the location of a sunbeam on the floor of a room with a skylight every 24 hours over the course of a year results in a shape known as the analemma. The analemma was an important navigational tool in the pre-GPS world, and was commonly included on maps of the time. It is still useful in areas such as architecture, solar energy engineering, landscaping, photography, and outdoor graphic design.