Download The Seasons (tropics version)

The Seasons (tropics version) - introduction
https://dept.astro.lsa.umich.edu/ugactivities/Labs/seasons/SeasonsIntrotr...
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Version: tropics intro
The Seasons
I must hear wise talk of the kind of weather, sort of season and
time of year
--Robert Browning
Overview
Explore the reasons for the seasons.
Understand the Sun's visibility and altitude at different latitudes and seasons.
Figure 1: Major celestial reference lines as they appear in the planetarium. You will need to be familiar with these.
Introduction
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The Seasons (tropics version) - introduction
https://dept.astro.lsa.umich.edu/ugactivities/Labs/seasons/SeasonsIntrotr...
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.
Figure 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.
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The Seasons (tropics version) - introduction
https://dept.astro.lsa.umich.edu/ugactivities/Labs/seasons/SeasonsIntrotr...
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.
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.
Resources
Sun Earth Geometry: http://www.physicalgeography.net/fundamentals/6h.html
Earth's orbit, seasons, special latitudes, and related information, including some activities you can do at home from the ASP
and Universe in the Classroom: http://www.astrosociety.org/education/publications/tnl/29/29.html
Official dates of Equinoxes, Solstices, Perihelion, and Aphelion, 2000-2020: http://aa.usno.navy.mil/data/docs
/EarthSeasons.php
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