Online Astronomy eText: The Sky
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Although this page is currently primarily a placeholder, with almost nothing posted for most of the topics to be covered, there are some useful diagrams part-way down, and in the next iteration of the page it will be considerably fleshed out.

Introduction
      (to be inserted: review of apparent annual motion of the Sun, and its relation to the seasons)

The Orbital Motion of the Earth
      (discussion of motion of Earth around the Sun, and how it would appear to move as seen from the Sun)

The Apparent Motion of the Sun
      (discuss the motion of the Sun along the Ecliptic, and its relationship to our orbital motion show how, because of our orbital motion the Sun moves to the East among the stars and how, because of our tilt it moves North and South during the year)
An image showing the position the Sun rises on the eastern horizon at different times during the year
     The north-south motion of the Sun, shown by its position at sunrise. On the first day of spring (the Vernal Equinox) the Sun rises due (exactly) East; on the first day of summer (the Summer Solstice) it has moved well to the north (23 1/2 degrees relative to the Celestial Equator, and that much or more along the horizon, depending on your latitude); and on the first day of winter (the Winter Solstice) it has moved well to the south (the same angle south of East that it was north of East at the Summer Solstice). (Anthony Ayiomamitis, apod051221)

Seasonal Stars
      (show how the motion around the Sun causes the stars which are visible at different times of night to gradually change throughout the year emphasize that this change would occur regardless of how we are tilted)

Rotation and Day Length
      (brief discussion of the way in which the eastward motion of the Sun produces a difference between the diurnal motions of the stars and of the Sun, and a statement that this difference causes the day and the rotation to be different, with a link to Rotation Period and Day Length)

The Cause of the Seasons
      (review of how the stars move as seen at different latitudes, and brief discussion of how the north-south motion of the Sun would change its diurnal motion during the year)

The View From Space
      (explanation of the seasons as seen from space)
Diagram showing how the motion of the Earth around the Sun tilts the Northern hemisphere toward the Sun on one side of our orbit, and away from the Sun on the other side of our orbit
The motion of the Earth around the Sun, showing how the Northern hemisphere is tilted toward the Sun on one side of the orbit and away from the Sun on the other side of the orbit. (Wikipedia Commons)

Diagram showing how the paths represented by the parallels of latitude are cut by the terminator (the line between day and night) at the Equinoxes in March and September
A more detailed view showing various parallels of latitude at the Equinoxes (the view is the same at both Equinoxes). The Earth's axis appears vertical in theis view, but is actually tilted toward or away from the observer by 23 1/2 degrees. As the Earth rotates, everyone moves to the East along their parallel of latitude, several of which are shown. Every one of the parallels is cut exactly in half by the terminator, the line dividing the day side of the Earth (on the left) from the night side (on the right), so everyone experiences half a day of sunlight and half a day of night. If the Earth had no tilt, this view would show the motions at various latitudes all year long, and everyone would have half a day of daylight and half a day of night every single day. As a result, there would be no "seasonal" changes in the path of the Sun, or our weather. (Przemyslaw Idzkiewicz, Wikipedia Commons)

Diagram showing how the paths represented by the parallels of latitude are cut by the terminator (the line between day and night) at the Northern Hemisphere's Summer Solstice in June
A similarly detailed view, at the summer solstice. The tilt of the Earth's axis (shown, unfortunately, as if the observer is on the back side of the first diagram) puts the North Pole in "permanent" daylight (it isn't going anywhere as the Earth rotates). The Equator, however, is still cut exactly in half by the terminator, and experiences half day and half night. Moving from the Equator toward the North Pole, more and more of the parallels of latitude are in daylight and less and less in darkness, so the days get longer and the nights shorter, until at the Arctic Circle the Sun is visible all day long. Going from the Equator toward the South Pole, less and less of each parallel is in daylight and more and more in darkness, so the days get shorter and the nights longer until it is dark all day long at the Antarctic Circle, and "permanent" night at the South Pole (which isn't going anywhere). (Przemyslaw Idzkiewicz, Wikipedia Commons)

The View From the Ground
      (explanation of the seasons, as seen from various places on the ground)

What If the Earth Were Not Tilted?
      (The view from space and from the ground, if the Earth were NOT tilted)

Summary
     
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