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Unit 2 The Sky and Celestial Motions PowerPoint PPT Presentation

Unit 2 The Sky and Celestial Motions. The sky and the constellations Daily motion of the Earth Annual motion of the Earth Motion of the Moon Eclipses Motions of the planets Precession. The European Southern Observatory in the Andes Mountains of Chile. A Model of the World. Cosmology.

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Unit 2 The Sky and Celestial Motions

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Unit 2The Sky and Celestial Motions

  • The sky and the constellations

  • Daily motion of the Earth

  • Annual motion of the Earth

  • Motion of the Moon

  • Eclipses

  • Motions of the planets

  • Precession

The European Southern Observatory in the Andes Mountains of Chile

A Model of the World


  • Cosmology is the study of the structure and evolution of the Universe.

  • Ancient world models represent the earliest cosmologies.

  • Ancient Egyptians and Babylonians both thought that Earth is flat and the sky is a dome arching above it.

  • See also the Figures on p. 62 of Bless.

The Horizon System

  • The horizon

  • The cardinal directions, N, S, E, W

  • The point directly above is the zenith

  • The point directly below is the nadir

  • Imagine a sphere all around the sky on which are “pinned” the sun, moon, planets and the stars -- this is the celestial sphere

  • Coordinates altitude and azimuth

The Daily Motion of the Earth

  • Earth spins on its axis.

  • This causes the change of day and night, and the rising and setting of the Sun, stars, Moon and planets.

  • The extensions of the Earth’s spin axis onto the celestial sphere mark the north celestial pole and the south celestial pole.

The Equatorial System

  • The zenith and the nadir

  • The (Earth) celestial poles

  • The (Earth) celestial equator

  • Latitudes north and south of equator (on Earth) - Declination on the celestial sphere

  • Longitudes east and west of the Greenwich meridian (on Earth) or the spring equinox (on the celestial sphere - Right Ascension)

The Annual Motion of the Earth

  • Earth revolves around the Sun.

    • The Sun appears to move constantly eastward among the stars.

  • The Sun blocks out different constellations throughout the year.

  • We see different constellations in the night sky during different seasons.

  • The apparent path of the Sun through the sky is called the ecliptic. The constellations located along the ecliptic are the constellations of the zodiac.

The Sun -- a quick Summary

Equinoxes and Solstices

  • The apparent path of the Sun through the constellations, the ecliptic, crosses the celestial equator in two points. These are the equinoxes.

    • On the day of the spring/fall equinox, the sun rises in the east and sets in the west. Day and night have equal length.

  • The Sun is furthest away from the celestial equator on the days of the solstices.

  • During the day of the summer solstice, the Sun rises due N of E and sets due N of W. It reaches the highest point in the sky. This is the longest day of the year.

  • During the day of the winter solstice, the Sun rises due S of E and sets due S of W. It makes its shortest and lowest arc through the sky. This is the shortest day and the longest night of the year.

  • The time from one vernal equinox to the next is called the tropical year.

  • The Seasons

    • The reason for the seasons is the tilt of Earth’s spin axis by 23.5o with respect to its orbital plane, the ecliptic.

    • During summer on the northern hemisphere, the northern half of Earth is tilted toward the Sun. The northern hemisphere has longer days.

    • Sunlight strikes the ground more from overhead, heating it up.

    The Motion of the Moon

    • The Moon spins on its axis with the same rate as it orbits around the Earth.

    • Therefore, the Moon always shows us the same face.

    • The length of the month derives itself from the lunar phase cycle.

    Measuring the Positions of Celestial Objects

    • Angular separation, measured from the observer, is the angle between lines toward two objects.

    • A minute of arc is one-sixtieth (1/60) of a degree of arc.

    • A second of arc is one-sixtieth (1/60) of a minute of arc.

    The apparent sizes of the Sun and the Moon in the Sky

    • The previous figure shows a person looking at two objects of very different size.

    • To the observer, however, they appear to be equally large. In fact, the small nearby object will exactly hide the more distant large object.

    • Because we cannot judge distances when we see astronomical bodies, what we call their sizes in the sky are actually their angular extents.

    • The Moon is 400 times smaller than the Sun, but because the Sun is also 400 times farther from the Earth than the Moon, they appear to be equally big to us. Both subtend (i.e. occupy an angle) half a degree.

    • This cosmic coincidence allows the occurrence of total eclipses of the Sun.

    • To get the true physical size, we must also know an objects distance from us.


    • Due to a cosmic coincidence, the the Sun and the Moon have equal angular sizes as seen from Earth.

    • Solar eclipses occur when the Moon moves between Sun and Earth.

      • The phase of the Moon is new.

      • Solar eclipses can be viewed only from locations in the Sun’s shadow path.

      • Thus, Solar eclipses are rare to observe from a given place on earth.

    • Lunar eclipses happen when the Earth moves between the Sun and the Moon.

      • The phase of the Moon is full.

      • Lunar eclipses can be viewed from an entire hemisphere of Earth.

      • They may be observed more frequently from a given location on Earth.

    Predicting Eclipses

    • The plane of the lunar orbit is inclined by 5o with respect to the plane of Earth’s orbit.

    • Eclipses happen when the Moon is located in its orbit exactly at the crossing point with the Earth’s orbit; this is called a node.

    • In addition, the phase must be full or new.

    • The time elapsed from ascending to ascending node is called a draconitic month.

    The Saros Cycle

    • Ancient peoples were able to predict eclipses not because they understood the orbits of the Earth and the Moon, but by noticing the Saros cycle.

    • The draconitic month is 27.21222 days.

    • The synodic month (the period from one full moon to the next) is 29.530959 days.

    • The Saros cycle has a length of 6,585 days, after which eclipses recur.

    • It is derived as the multiple of the draconitic and the synodic month. 242 draconitic months (6585.32 days) approximately equal 233 synodic months (6585.36 days).

    • The periodic recurrence of eclipses after about 18 yr 11 d and the discovery of the Saros cycle is credited to the Chaldaean (=Babylonian) people, and dates back to several hundred years BC.

    The Fate of Two Chinese Astronomers

    • The occurrence of eclipses has (to this day) instilled fear in people.

    • The ancient Chinese had a lore that the Sun was swallowed by a dragon during an eclipse.

    • Chinese astronomy probably began < 2000 BC although details are largely legendary.

    • The story of two astronomers, Ho and Hi, executed for failing to predict a solar eclipse in 2137 BC may not be true and may refer to two astronomical colleagues of a much later date who died in a civil unrest.

    Path of Mars from August 1 to October 1, 2003 through the Constellations Capricornus, Aquarius, and Pisces

    Planetary Motions

    • In addition to the daily motion and the annual motion of the Earth, the planets show motions of their own among the constellations.

    • The word planet derives itself from the Greek word for “wanderer”, implying that the planets move with respect to the fixed stars.

    • A planet’s motion eastward is called direct as it follows the motions of the Sun and the Moon across the sky.

    • The westward, looping motion of the planets is called retrograde.

    • It poses a great challenge for Earth-centered cosmologies (discussed later and revolutionized with Copernicus’s view of a Sun-centered solar system).

    • The apparent paths of the planets on the celestial sphere are slightly N and S of the ecliptic, but in the zodiac.

    • This is so because the planets’ orbits are a little, but not very much, tilted with respect to the Earth’s orbit.

    • The closer a planet is to the Sun, the faster the planet completes one revolution about the Sun.

    • The reason for the apparent motion of the planets on the celestial sphere is a combination of the Earth’s and their own orbital motion about the Sun.

    Planetary Configurations


    • The direction of Earth’s spin axis changes.

    • Earth’s spin axis completes a full circle in 26,000 yrs.

    • This is also called the platonic year.

    • Precession is caused by the gravitational attraction of the Moon and the Sun on the equatorial bulge of Earth.

    • It was discovered 150 BC by Hipparchus (see p. 84 of Bless).

    Effects of Precession

    • Because of precession, Polaris is the north star only temporarily; when the pyramids were built 5,000 yrs ago, the pole star was in the constellation of Draco; in 12,000 yrs, the north celestial pole will be near the bright star Vega.

    • Precession also causes the precession of the equinoxes; the vernal equinox moves into a different constellation about every 2,000 yrs.

    UNIT 2 Learning Objectives

    • Motions of the Earth, Sun and Moon, planets

    • The phases of the Moon

    • Lunar and solar eclipses, their prediction

    • The reason for the seasons

    • Equinoxes and solstices

    • Ancient and modern models for the sky

    • Astronomical coordinate systems

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