A second effect of precession is the shifting of the signs along the zodiac. The _zodiac_ is a belt of the heavens along the ecliptic, extending eight degrees from it on each side. This belt is occupied by twelve constellations, known as the _zodiacal constellations_. They are _Aries_, _Taurus_, _Gemini_, _Cancer_, _Leo_, _Virgo_, _Libra_, _Scorpio_, _Sagittarius_, _Capricornus_, _Aquarius_, and _Pisces_. The zodiac is also divided into twelve equal parts of thirty degrees each, called _signs_. These signs have the same names as the twelve zodiacal constellations, and when they were first named, each sign occupied the same part of the zodiac as the corresponding constellation; that is to say, the sign Aries was in the constellation Aries, and the sign Taurus in the constellation Taurus, etc. Now the signs are always reckoned as beginning at the vernal equinox, which is continually shifting along the ecliptic; so that the signs are continually moving along the zodiac, while the constellations remain stationary: hence it has come about that the _first point of Aries_ (the _sign_) is no longer in the _constellation_ Aries, but in Pisces. [Illustration: Fig. 37.] Fig. 38 shows the position of the vernal equinox 2170 B.C. It was then in Taurus, just south of the Pleiades. It has since moved from Taurus, through Aries, and into Pisces, as shown in Fig. 39. [Illustration: Fig. 38.] [Illustration: Fig. 39.] Since celestial longitude and right ascension are both measured from the first point of Aries, the longitude and right ascension of the stars are slowly changing from year to year. It will be seen, from Figs. 38 and 39, that the declination is also slowly changing. 30. _Nutation._--The gyratory motion of the earth's axis is not perfectly regular and uniform. The earth's axis has a slight tremulous motion, oscillating to and fro through a short distance once in about nineteen years. This tremulous motion of the axis causes the pole of the heavens to describe an undulating curve, as shown in Fig. 40, and gives a slight unevenness to the motion of the equinoxes along the ecliptic. This nodding motion of the axis is called _nutation_. [Illustration: Fig. 40.] 31. _Refraction._--When a ray of light from one of the heavenly bodies enters the earth's atmosphere obliquely, it will be bent towards a perpendicular to the surface of the atmosphere, since it will be entering a denser medium. As the ray traverses the atmosphere, it will be continually passing into denser and denser layers, and will therefore be bent more and more towards the perpendicular. This bending of the ray is shown in Fig. 41. A ray which started from _A_ would enter the eye at _C_, as if it came from _I_: hence a star at _A_ would appear to be at _I_. [Illustration: Fig. 41.]