University of Michigan - Department of Astronomy

Version: tropics intro

The Seasons

I must hear wise talk of the kind of weather, sort of season and time of year

--Robert Browning


Figure 1: Major celestial reference lines as they appear in the planetarium. You will need to be familiar with these. Click the image for a larger view.Celstial sphere


Why does the Earth experience seasons? How does the tilt of the Earth's axis affect the length of day and night? How does it define the tropical and arctic regions? The tilt of the Earth's axis relative to its orbital motion causes the amount of daylight falling on the northern versus southern hemispheres to vary as the Earth orbits around the Sun. In thinking about this, our main reference is the ecliptic, which is the plane of Earth's orbit around the Sun. From the Sun's point of view, the ecliptic is the Earth's path against the background stars. From the Earth's point of view, this exact same path, the ecliptic, corresponds to the Sun's apparent track in our sky, against the background stars through the year; it is labeled in Figure 1 above and in the planetarium. The background constellations that occupy this path are those of the zodiac

Seasons and Earth's orbitFigure 2: Northern seasons resulting from the Earth's tilt relative to its orbit. Using the dotted lines, we can find the latitude at which a person sees the Sun overhead on the given day. Since a person stands perpendicular to the Earth's surface, "overhead" is the direction perpendicular to the surface (see also Figure 3).


Figure 2 illustrates the northern seasons resulting from the Earth's tilt relative to its orbit. The summer and winter solstices are the points at which the Earth's pole is aligned, respectively, toward or away from the Sun. Note that "toward" and "away" depend on whether you live in the northern or southern hemisphere! The spring (vernal) and autumn equinoxes are the midway points between the solstices, when the Earth's axis is aligned perpendicular to the Sun's direction. Thus, night and day are of equal length on the equinoxes. Note that the equinoxes also correspond to the points on the sky where the ecliptic (the Sun's path) intersects the celestial equator (see Figure 1). The March equinox is defined to be the zero-point for both Right Ascension (which is measured along the celestial equator), and angular distance around the ecliptic.

Following the dotted lines in Figure 2, you will see that on the equinoxes, a person standing at the equator will see the Sun directly overhead; the Sun's direction is perpendicular to the Earth's surface at that location. The Tropic of Cancer is the northern-most latitude at which an observer can see the Sun directly overhead. As we see in Figure 3, this occurs only once a year, at noon on the June solstice, when the Sun is at its northern-most position in the sky. Because the Earth's rotational axis is tilted by 23.5º relative to its orbit, this latitude corresponds to 23.5º N latitude (see Figure 3). Similarly, the Tropic of Capricorn , at 23.5º S latitude, is where an observer would see the Sun directly overhead at noon on the December solstice, which is the southern summer solstice. The tropics were named for the constellation toward which the Sun was located on the solstices, roughly 4000 years ago. Due to precession, (see the Precession activity) the Sun is no longer in those constellations on the solstices today.

Figure 3: The Tropics of Cancer and Capricorn, arctic and antarctic circles. These latitudes are defined by the Sun's seasonal positions in the sky. The zenith is observed in the direction perpendicular to the Earth's surface. For example, on the December solstice, an observer on the Tropic of Capricorn would see the Sun on the zenith at noon.Seasons and Earth's orbit

Also on the summer solstice, the Sun can be seen on the horizon at midnight, at the latitude of the arctic or antarctic circles, as seen in Figure 3. On these days, the Sun never sets. You know that in the equatorial coordinate system, we project the Earth’s poles and equator onto the sky to give us the north and south celestial poles and celestial equator. Similarly, the Tropics of Cancer and Capricorn, and the arctic and antarctic circles may be projected onto the sky for reference, in celestial maps or atlases.



Last modified: 08/28/14 by SAM and MSO. Additional material from EMP.

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