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Version: tropics intro |
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I must hear wise talk of the kind of weather, sort of season and time of year
--Robert Browning
Please study Figure 1 in color to familiarize yourself with these features in the planetarium.
Figure 1: Major celestial reference lines as they appear in
the planetarium. Click the image for a larger view.
Seasonal behavior is caused by the changing position of the Earth in its orbit around the Sun, combined with the tilt of the Earth's rotational axis about the poles. From the Sun's point of view, the ecliptic is the path of Earth's orbit around the Sun, seen 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 the 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 the 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.
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.
In this activity, you will explore how the Sun's position and apparent motion vary seasonally and depend on the observer's latitude. Before class, please review Figure 1 and the following terms from the Coordinate Systems activity.
You will be using the local sidereal time (LST; see the Timekeeping and Telescopes at the Detroit Observatory Activity) as an approximation of solar time. In the planetarium, the LST is determined by reading the hour on the equator that is crossing the meridian (see Figure 1). There are tick marks every 10 minutes, with longer marks for the hour and half hour.
The date is based on the Sun's position on the ecliptic. In the planetarium, the ecliptic is labeled with the calendar dates, but watch the direction: the Sun moves eastward through the stars! Note that the position of the Sun is only accurate to about 2 days (e.g., the computer may set the Sun's position for March 21, but the Sun may actually appear to sit on March 19.) When asked for the date, record the position of the Sun that you observe, but keep this inaccuracy in mind when doing the activity.
The cardinal directions (N, E, S, W) are azimuthal directions on the horizon, where the north and south cardinal points are defined by the intersection of the meridian with the horizon (Figure 1). East and west are defined to be 90 and 270 degrees clockwise on the horizon, when facing north, which is 0 degrees azimuth. When facing down toward the Earth, east is to the right of north. However, when facing the sky, east will appear to the left of north, instead of to the right!
Azimuth is measured in degrees from the north cardinal point on the horizon (0 degrees azimuth), toward east. It is represented by the horizontal arrow in Figure 1. The azimuthal direction can be given roughly, using the cardinal directions. For example, an object's azimuthal direction can be estimated as north-northeast (NNE), or west-southwest (WSW). In this activity, the azimuthal direction is used to determine rising and setting directions for celestial objects.
Altitude is measured in degrees above the horizon, and is represented by the vertical arrow in Figure 1. It is most easily measured when an object is on the meridian because the meridian is exactly perpendicular to the horizon. In the planetarium, it is conveniently marked in degrees. Please note: The wall in the planetarium cuts off the horizon 2 degrees above the true horizon.
Last modified: 8/24/09 by SAM and MSO. Additional material from EMP.
Copyright Regents of the University of Michigan.