the constellations seem to move as the seasons on earth change. what causes this?

Southo far we have explained the paths that stars on the celestial sphere announced to take through the local sky each day. Merely why do we see different sets of stars — that is, dissimilar constellations — at dissimilar times of year? The answer is that the night sky changes because of Earth's orbit around the Sun, and as you'll run across, this also explains the path called the ecliptic on the angelic sphere.

Call back that the celestial sphere is an illusion created by our lack of depth perception in infinite. In reality, Earth orbits the Sun equally shown in Effigy 2.sixteen, while the stars that dot the celestial sphere are all much farther away (and themselves located at different distances from Earth). However, because we can't tell but by looking that the stars are then far away, Figure 2.16 imagines them located just beyond Globe'southward orbit of the Sun. Moreover, because Globe follows the same orbital path twelvemonth after year, we always run across the Dominicus appearing to motion amid the same set up of constellations — chosen the constellations of the zodiac — from one year to the side by side.

Detect that, from our vantage signal on Earth, nosotros see the Sun appear to movement steadily on an east path through the constellations of the zodiac. Retrieve that this path is called the ecliptic, and this is why the ecliptic is marked on the celestial sphere (equally nosotros first saw in Effigy 2.half dozen).

To run into how Earth'due south orbit leads us to see different constellations at different times of yr, follow along as y'all look at Effigy 2.16:

• As you look at Earth's orbit around the Sun, notice the position labeled March 21, which represents where Globe is located each twelvemonth on that date.
• Find that if you draw a line from that position through the Sun, the Sunday appears to be on the celestial sphere at the location of the yellow dot in the constellation Pisces on that date. This means that we won't be able to run into Pisces at night on March 21, because it volition be in the daytime heaven forth with the Sun.
• But if nosotros look in the reverse direction, following the "dark" arrow, we'll run across the constellation Virgo at the bespeak on the celestial sphere that is exactly opposite the Sun. This means we'll see Virgo ascent at dusk, reaching the meridian at midnight, and setting at sunrise.
• If yous pick whatever other appointment, you'll detect the Sun in a different location along the ecliptic, and therefore volition meet different constellations in your nighttime heaven.

Figure 2.16 – The Sun appears to move steadily eastward along the ecliptic as Earth orbits the Lord's day, so we see the Sun against the groundwork of different zodiac constellations at dissimilar times of yr. For example, on March 21 the Sun appears to be in Pisces, because it is between the states and the much more than distant stars that make up Pisces. Credit: The Catholic Perspective.

Activity

Tonight's Sky

What is today's date? Use Figure 2.xvi to find the name of the zodiac constellation in which the Sun currently appears. What constellation of the zodiac volition be on your meridian at midnight tonight? Name at least iv other zodiac constellations that volition be visible in your heaven at midnight.

Teacher Notes: For this activeness, we've focused on midnight because it is the easiest to read from Effigy two.16. At midnight, you'll certainly be able to run into the two zodiac constellations on either side of the one that is directly opposite the Sun. If yous want to figure out what you'll see at other times, remember that at any given moment you can run across half the celestial sphere, which ways roughly half of the zodiac. The caveat is that the fact that ecliptic is tilted to the celestial equator ways that you rarely see exactly half of information technology in your sky, with the amount visible at whatever given moment depending on your latitude and the appointment and time.

Connections—Mathematics

Why are there 360° in a Circle

The selection of 360° for a full circle goes dorsum to cultures of the aboriginal Middle East. No ane knows for sure why they chose 360 rather than some other number to marking a full circle, simply information technology seems very likely that ane reason was the fact that the Sun takes a year to motility around the total circle of the ecliptic, and 360 days is a adequately close approximation to a year. Indeed, some ancient calendars used a year with 360 days, in part because the Moon's cycle of phases is very close to 30 days (run into Section 2.3), then it seemed mathematically natural to have a yr consist of 12 months of 30 days each. Aboriginal mathematicians also noticed other special properties of the number 360 — for case, it has 24 divisors, including every number from 1 to x except vii — which may have added to a sense that 360 was an "of import" number and therefore a good choice for the number of degrees in a circle.

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