# Orientation in the Sky and Constellations Revision Notes

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22.8Orientation in the Sky and Constellations

In these revision notes for Orientation in the Sky and Constellations, we cover the following key points:

• What is celestial sphere?
• What is the configuration of celestial sphere?
• What are equatorial coordinates? How to calculate them?
• What is horizon?
• What are horizontal coordinates? How to calculate them?
• What are vernal points? When do they occur?
• What are constellations? Which are the most visible constellations in the sky?
• Where do constellations differ from galaxies?
• What are the Zodiac constellations? Do they have any connection to the fate of people?

## Orientation in the Sky and Constellations Revision Notes

We consider (for convenience) the sky as a very large sphere where all stars lie on its surfaces with the Earth located at its centre, this sphere is known as the celestial sphere. We know that the Earth rotates around its axis from West to East. Hence, we assume the celestial sphere as rotating from East to West.

The rotation axis PP' of celestial sphere is known as the "world axis"; the points where this axis punch the celestial sphere are known as "world poles", namely north and south pole. Stars perform a circular motion during the rotation of the celestial sphere. The perpendicular plane to world axis is called the equatorial plane. Its intersection with the celestial sphere is called the celestial equator.

The most used coordinates for representing the objects position in the celestial sphere are the equatorial coordinates. The equatorial coordinate system is basically the projection onto the celestial sphere of the latitude and longitude coordinate system we use here on Earth. By direct analogy, lines of latitude become lines of declination (Dec; measured in degrees, arcminutes and arcseconds) and indicate how far north or south of the celestial equator (defined by projecting the Earth's equator onto the celestial sphere) a celestial body lies. Lines of longitude have their equivalent in lines of right ascension (RA), but unlike in geography, where longitude is measured in degrees, minutes and seconds east the Greenwich meridian, RA here is measured in hours, minutes and seconds east from where the celestial equator intersects the ecliptic (the vernal equinox or vernal point Vp).

Horizon is the intersection of the plane tangent with Earth surface and the celestial sphere. We say a star rises when it appears above the horizon (in the evening) and it sets when disappears from the horizon (in the morning). The highest position of a star in the sky is called the zenith. It is at mid-path of the star's trajectory from east to west in the sky. The star makes the same trajectory from west to east as well, but an observer cannot see this movement because it occurs from the other side of the globe. When the star is at mid-path of this invisible trajectory, we say it is in the nadir (the opposite of zenith).

The position of a star can also be determined in reference to the horizon; in this instance, we use the horizontal coordinates to express the position of a star.

Ecliptics is a big circle of celestial sphere obtained by its intersection with the orbital plan of the Earth revolution around the Sun, which is at different points of ecliptics in various periods of year. The values of right ascension and declination of the Sun, planets and Moons are never the same during a year. We have to wait until the same day of the following year to obtain the same identical values of RA and Dec.

Vernal points represent the intercept of ecliptic with the celestial sphere. There are two vernal points, which correspond to the two equinoxes (spring and autumn ones).

Some of the most visible stars that appear close to each other form different patterns, which when we connect them with our mind, form imaginary figures called constellations. They are important to help us find navigate during moonless nights. There are, in total, 88 constellations that divide the sky into regions according to the most confirmed figures formed in the sky.

The most visible constellation present in the night sky is Ursa Major which lies in the northern sky. Ursa Major consists of 7 stars. Ursa Minor is another constellation very close to Ursa Major. Between Ursa Major and Ursa Minor there is a long series of stars that form a bent line known as Draco (Dragon) Constellation.

The Zodiac is a belt of stars comprising 12 constellations located near the ecliptics in the celestial sphere. The Latin names of Zodiac constellations with their meaning in English given inside braces are Aries (Ram), Taurus (Bull), Gemini (Twins), Cancer (Crab), Leo (Lion), Virgo (Virgin), Libra (Balance), Scorpius (Scorpion), Sagittarius (Archer), Capricorn (Goat), Aquarius (Water bearer) and Pisces (Fishes). The following figure shows all constellations contained in the Zodiac Belt.

The combination in positions of the Sun, Moon, planets and stars in the Zodiac constellations is misused by certain individuals (astrologers) who claim to know the future of people by "reading" the hidden messages these celestial bodies reveal to humanity through their alignment in the sky. They have invented the "12 Zodiac Signs" that divide people in 12 groups according their birthday. Astrologers claim that the fate of people is related to the position of stars in the sky. Obviously, such claims are complete nonsense, as no sound mind can accept the idea that the fate of humans is related to the position of celestial bodies. Astrology (the pseudo-science that deals with the relationship between humans future and celestial bodies) has no scientific base and the term "astrology" has nothing to do with astronomy - the science dealing with the study of celestial bodies.

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