Let us begin following the Sun during the course of the year starting at the March equinox. As the days pass during the months of April, May, and June, the Sun stays above the horizon for less and less of each day, and it follows a progressively lower course across the sky. The change is rapid in the first days after the equinox, and becomes more gradual with the approach of the June solstice, which takes place on around June 22 give or take a day. This might be called the “winter solstice,” but again, to avoid confusion with northern-hemisphere-based observers who call it the “sum- mer solstice,” it is better to name the month in which it occurs.
At the June solstice, the Sun has reached its northernmost declination point, approximately dec = +23.5 degrees. The Sun has made one-quarter of a complete circuit around its annual “lap” among the stars and sits at RA = 6 h. This situation is shown in Fig. 3-3 using the same two az/el coordinate schemes as those in Fig. 3-2. The gray line represents the Sun’s course across the sky. As in Fig. 3-2, the time of day is midafternoon. The observer’s geographic latitude is the same too: 35°S.
North West
South
East
60 30 0
Sun
B
0 60
West East
South North
30
Sun
A
Figure 3-3. Az/el sky maps for midafternoon at 35 degrees south latitude on or around June 21.
After the June solstice, the Sun’s declination begins to decrease, slowly at first and then faster and faster. By late September, the other equinox is reached, and the Sun is once again at the celestial equator, just as it was at the March equinox. But now, instead of moving from south to north, the Sun is moving from north to south in celestial latitude. At the September equinox, the Sun’s RA is 12 h. This corresponds to 180 degrees.
Now it is the spring season in the southern hemisphere, and the days are growing long. The Sun stays above the horizon for more and more of each day, and it follows a progressively higher course across the sky. The change is rapid during September and October and becomes slower and slower with the approach of the December solstice, which takes place on December 21, give or take a day.
At the December solstice, the Sun’s declination is at its southernmost point, approximately dec = ⫺23.5 degrees. The Sun has gone through three-quarters of its annual “lap” among the stars, and sits at RA=18 h. This is shown in Fig. 3-4 using the same two az/el coordinate schemes as those in Figs. 3-2 and 3-3. The gray line represents the Sun’s course across the sky. As in Figs. 3-2 and 3-3, the time of day is midafternoon. The observer hasn’t moved either, at least in terms of geographic latitude; this point is still at 35°S.
After the December solstice, the Sun’s declination begins to increase gradually and then, as the weeks pass, faster and faster. By late March, the Sun reaches an equinox again and crosses the celestial equator on its way to forsaking the southern hemisphere for another autumn and winter. The
“lap” is complete.
Mirrored Myths
The Greeks didn’t name the southern circumpolar constellations, but many of the star groups near the equator, as seen from “down under,” are the same ones that the Greeks made famous. The only difference is that they are all upside down.
SKY MAPS
In this chapter, the general shapes of the better-known southern constella- tions are shown. To see where these constellations are in the sky from your
North West
South
East
60 30 0
Sun
B
60 0
West East
South North
30
Sun
A
Figure 3-4. Az/el sky maps for midafternoon at 35 degrees south latitude on or around December 21.
location this evening, go to the Weather UndergroundWeb site at the fol- lowing URL:
http://www.wunderground.com
Type in the name of your town and country, and then, when the weath- er data page for your town comes up, click on the “Astronomy” link. There you will find a detailed map of the entire sky as it appears from your loca- tion at the time of viewing, assuming that your computer clock is set cor- rectly and data are input for the correct time zone.
Southern Circumpolar Constellations
From the latitude of 35°S, the circumpolar constellations encompass much of the sky. At some time or other during the year, it is possible to see more of the sky at lower latitudes (closer to the equator) than at higher latitudes (closer to the poles). If you live in Sydney, Buenos Aires, or Cape Town, you have a slight advantage in this respect over your counterparts who live in Minnesota and a bigger advantage over people in Scotland. However, the portion of the sky that stays above the horizon, no matter what time of the year you stargaze in the evening sky, becomes smaller as you go closer to the equator. Observers in chillier climes get to see more circumpolar constellations but less of the complete celestial sphere; people in warmer places get to see more of the celestial sphere, but they have to choose the proper times to see specific con- stellations near the pole.
STAR BRIGHTNESS
In this chapter, as in Chapter 2, stars are illustrated at three relative levels of brightness. Dim stars are small black dots. Stars of medium brilliance are larger black dots. Bright stars are circles with black dots at their cen- ters. But the terms dim,medium, and brightare not intended to be exact or absolute. In downtown Sydney, some of the dim stars shown in these draw- ings are invisible, even under good viewing conditions, because of scat- tered artificial light. After your eyes have had an hour to adjust to the darkness on a moonless, clear night in the outback, some of the dim stars in these illustrations will be easy to see. The gray lines connecting the stars
are included in the diagrams only to emphasize the general shapes of the constellations.