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Constellations on the Ecliptic
Constellation on the Ecliptic Dates When the Sun Crosses It
Capricornus January 21–February 16
Aquarius February 16–March 11
Pisces March 11–April 18
Aries April 18–May 13
Taurus May 13–June 22
Gemini June 22–July 21
Cancer July 21–August 10
Leo August 10–September 16
Virgo September 16–October 31
Libra October 31–November 23
Scorpius November 23–November 29
Ophiuchus November 29–December 18
Sagittarius December 18–January 21

The ecliptic does not lie along the celestial equator but is inclined to it at an angle of about 23.5°. In other words, the Sun’s annual path in the sky is not linked with Earth’s equator. This is because our planet’s axis of rotation is tilted by about 23.5° from a vertical line sticking out of the plane of the ecliptic    ( [link] ). Being tilted from “straight up” is not at all unusual among celestial bodies; Uranus and Pluto are actually tilted so much that they orbit the Sun “on their side.”

The celestial tilt.

The Tilt of the Celestial Equator. At the center of the figure the Earth is drawn with the North and South poles labeled and the Equator drawn as a black line. An observer is shown standing in the Northern Hemisphere. The Earth is embedded in a sphere representing the sky. A line is drawn vertically upward from the observer to the sphere, and is labeled “Line perpendicular to ecliptic”. Another line is drawn projecting from the North Pole to the sphere. An angle is drawn between the “Line perpendicular to ecliptic” and the projected North Pole line and labeled “23 ½ degrees”. The Earth’s equator is projected onto the sky and drawn as a white dashed circle and labeled “Celestial equator”. The ecliptic is drawn on the sphere as red circle. Another angle is drawn between the ecliptic and the celestial equator and labeled “23 ½ degrees”. Finally, the Sun is drawn in four locations throughout the year on the ecliptic circle. Two are in June and December when the Sun is farthest from the celestial equator (the solstices), and two are in March and September when the Sun is on the points where the ecliptic and celestial equator meet (the equinoxes).
The celestial equator is tilted by 23.5° to the ecliptic. As a result, North Americans and Europeans see the Sun north of the celestial equator and high in our sky in June, and south of the celestial equator and low in the sky in December.

The inclination of the ecliptic is the reason the Sun moves north and south in the sky as the seasons change. In Earth, Moon, and Sky , we discuss the progression of the seasons in more detail.

Fixed and wandering stars

The Sun is not the only object that moves among the fixed stars. The Moon and each of the planets that are visible to the unaided eye—Mercury, Venus, Mars, Jupiter, Saturn, and Uranus (although just barely)—also change their positions slowly from day to day. During a single day, the Moon and planets all rise and set as Earth turns, just as the Sun and stars do. But like the Sun, they have independent motions among the stars, superimposed on the daily rotation of the celestial sphere. Noticing these motions, the Greeks of 2000 years ago distinguished between what they called the fixed stars —those that maintain fixed patterns among themselves through many generations—and the wandering stars , or planets . The word “planet,” in fact, means “wanderer” in ancient Greek.

Today, we do not regard the Sun and Moon as planets, but the ancients applied the term to all seven of the moving objects in the sky. Much of ancient astronomy was devoted to observing and predicting the motions of these celestial wanderers. They even dedicated a unit of time, the week, to the seven objects that move on their own; that’s why there are 7 days in a week. The Moon, being Earth’s nearest celestial neighbor, has the fastest apparent motion; it completes a trip around the sky in about 1 month (or moonth ). To do this, the Moon moves about 12°, or 24 times its own apparent width on the sky, each day.

Angles in the sky

A circle consists of 360 degrees (°). When we measure the angle in the sky that something moves, we can use this formula:

speed = distance time

This is true whether the motion is measured in kilometers per hour or degrees per hour; we just need to use consistent units.

As an example, let’s say you notice the bright star Sirius due south from your observing location in the Northern Hemisphere. You note the time, and then later, you note the time that Sirius sets below the horizon. You find that Sirius has traveled an angular distance of about 75° in 5 h. About how many hours will it take for Sirius to return to its original location?

Solution

The speed of Sirius is 75 ° 5 h = 15 ° 1 h . If we want to know the time required for Sirius to return to its original location, we need to wait until it goes around a full circle, or 360°. Rearranging the formula for speed we were originally given, we find:

time = distance speed = 360 ° 15°/h = 24 h

The actual time is a few minutes shorter than this, and we will explore why in a later chapter.

Check your learning

The Moon moves in the sky relative to the background stars (in addition to moving with the stars as a result of Earth’s rotation.) Go outside at night and note the position of the Moon relative to nearby stars. Repeat the observation a few hours later. How far has the Moon moved? (For reference, the diameter of the Moon is about 0.5°.) Based on your estimate of its motion, how long will it take for the Moon to return to the position relative to the stars in which you first observed it?

Answer:

The speed of the moon is 0.5°/1 h. To move a full 360°, the moon needs 720 h: 0.5 ° 1 h = 360 ° 720 h . Dividing
720 h by the conversion factor of 24 h/day reveals the lunar cycle is about 30 days.

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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