Some Facts about the Earth and the Sun

1. Some Facts about the Earth and the Sun Viewed from space, rays of solar radiation striking all parts of the earth come from the same direction (that is, they are parallel to each other). So, a ball (sphere) such as the earth, illuminated by parallels rays of solar radiation, will always be half lit by the sun and half dark. The boundary between the lit and dark halves of the earth is called the circle of illumination. The earth is a sphere, so the earth’s surface faces in a different direction everywhere on the earth. So, the angle between the sun’s rays and the earth’s surface varies from place to place. In particular, averaged over a whole year, the maximum sun angle at each latitude is smaller the farther you are from the equator.

3. Some Facts about the Earth and the Sun Repeat: Averaged over a whole year, the maximum sun angle at each latitude is smaller the farther you are from the equator. So, can the decrease in sun angle with increasing latitude help explain the global, spatial pattern of temperature that we observe?

4. Some Facts about the Earth and the Sun The earth rotates around it’s axis of rotation once in a day (24 hours). Over a 24-hour day, any particular place moves in a circle around the axis of rotation. That circle corresponds exactly to that place’s latitude circle. Every place on a latitude circle must correspond to a different time of day. At most latitudes, places will cross the circle of illumination twice each day, from the dark part of the earth to the light part (sunrise) and vice-versa (sunset). At most latitudes, the sun will reach a highest point in the sky half way between sunrise and sunset (at solar noon).

5. Some Facts about the Earth and the Sun The earth orbits (revolves around) the sun once per year. The earth’s axis of rotation tilts by 23.5° (relative to the plane of the earth’s orbit around the sun). The earth’s axis or rotation doesn’t change direction as the earth revolves around the sun—it always points toward the (very distant) North Star (Polaris).

6. Aspects of the diagram below that are accurate: • The earth’s axis of rotation points in the same direction (toward the distant North Start) all the way around the sun. • The side of the earth facing the sun is illuminated by the sun. • Aspects of the diagram that are not accurate: • The size of the earth and sun relative to each other and to the size of the earth’s orbit around the sun. • All of the labels (except “Sun”) are highly misleading! Can you tell why? • The degree to which the orbit is an ellipse is a exaggerated.

7. Some Facts about the Earth and the Sun The earth’s axis or rotation always points toward the (very distant) North Star (Polaris). This has consequences: At one point in the earth’s orbit around the sun, the axis of rotation it tilted most directly toward the sun. (This is one of the solstices.) On the opposite side of the orbit (6 months earlier or later), the axis of rotation is tilted most directly away from the sun. (This is the other solstice.) At the two points half way between the solstices, the axis of rotation tilts neither toward nor away from the sun, but off to the side. (These are the equinoxes. They are 3 months from each solstice.)

8. Some Facts about the Earth and the Sun The changing orientation of the earth relative to the sun has consequences: The angle of the sun above the horizon at solar noon varies with time of year. Outside the tropics, the sun angle is highest at the summer solstice and lowest at the winter solstice. (In the tropics, it’s more complicated.) The number of hours of daylight varies with time of year (except at the equator) Days are longest at the summer solstice and shortest at the winter solstice. Except: the equator always gets 12 hours of daylight.

9. Some Facts about the Earth and the Sun Sun angle and length of daylight varywith time of year, so daily average insolation at the earth’s surface varies with time of year, too. In particular, outside the tropics, daily average insolation is greatest at the summer solstice and is least at the winter solstice. Can these seasonal variations in insolation help explain observed seasonal variations in temperature with time of year? An observation: At middle latitudes in the N. Hemisphere on land, the warmest time of year is in July, and the coldest time of year is in January.(Over the oceans at the same latitudes, the warmest and coldest times are in August and February, respectively.)So, what do you think?