This set of five short videos simulate what the motions of the Sun would look like for an observer located on Earth viewing the sky. Imagine yourself standing on Earth as the observers in Figure 3.24 on page 79. You are encouraged to note rising/setting locations of the Sun, the altitude (angle) of the Sun above the horizon as it crosses the meridian, the changing length of the day, and the altitude of the north celestial pole (NCP) above the northern horizon with changing latitude and at various times of the year. Simulations at five latitudes are available: 1. the North Pole (+90 deg), 2. on the arctic circle (+66 1/2 deg), 3. in Salt Lake City, UT (+41 deg), 4. on the tropic of Cancer (+23 1/2 deg), and 5. on the equator (0 deg).
Each simulation also displays the motion of the Sun over a 24 hour period (color-coded paths) on the days when (a) the Sun crosses the vernal equinox (the Sun crosses the celestial equator traveling from south to north), (b) the Sun passes the summer solstice (its most northerly distance from the celestial equator before turning southward), (c) the Sun crosses the autumnal equinox (the Sun crosses the celestial equator traveling from north to south), and (d) the Sun passes the winter solstice (its most southerly distance from the celestial equator before turning northward). Each of the four days mark the first days of astronomical spring, summer, fall, and winter, respectively. Careful observation of the Sun in the simulations will also show that the Sun slips backward just under 1 deg per day along the ecliptic; note the position of the fixed marker on the ecliptic and the Sun’s position relative to it over the 24 period.
Between the four equinox and solstice days, the motion accelerates to mark only one position of the Sun each day (white marks), but at the same time each day. Over the course of the year the white marks will display a figure-eight-like shape (referred to as an analemma) that is due to the 23 1/2 deg tilt of the ecliptic to the celestial equator, and the uneven number of days between the four equinox and solstice days.
The available videos, based on latitude, are:
- North Pole (+90 deg)
- arctic circle (+66 1/2 deg)
- Salt Lake City, UT (+41 deg)
- tropic of Cancer (+23 1/2 deg)
- equator (0 deg)
All simulations were produced by the planetarium software: Stellarium (stellarium.org); GNU General Public License v3 (www.gnu.org/licenses/gpl-3.0.html). Audio will be available in a future update.