Grade 12 Physical Science: Models of the Universe

Models of the Universe

• The Greeks deduced that the Earth is spherical based on the observation that the shadow cast by the Earth during a lunar eclipse is circular and that the only shape that can cast a circular shadow at whatever direction is a sphere.
• Eratosthenes, a Greek Mathematician, measured the Earth's circumference by using the angles of shadows cast in two different cities during a solstice.
• The Earth's circumference is approximately 40,000 kilometers.

Astronomical Events Known to Men Before the Advent of Telescopes

• Ancient Babylonian, Assyrian, and Egyptian knew the length of the year and Egyptians adopted a calendar based on 365 days a year.
• The Egyptians also kept track of the yearly cycle of the star Sirius which corresponds to the flooding of Nile.
• Early Chinese civilizations kept track of the comets, meteors, and dark spots of the Sun.
• Mayan civilization developed a calendar based on the movements of Venus.
• The Polynesians utilized the stars for navigation.

Astronomical Events

• Diurnal motion: the apparent daily motion of stars and other celestial bodies across the sky due to Earth's rotation.
• Annual motion: the apparent yearly motion of stars and other celestial bodies across the sky due to Earth's revolution.
• Zodiac and the ecliptic: the path the sun takes in the celestial sphere, tracing the ecliptic, and a band of 13 constellations collectively called the zodiac.
• Equinoxes and solstices: the two days in a year in which the sun crosses the celestial equator, and the two days in a year in which the Sun is at the farthest declination from the celestial equator.
• Precession: the slow 'wobbling' of Earth's axis of rotation due to the gravitational pull of the Moon and Sun, resulting in a 26,000-year cycle.
• Eclipse: occurs when either the Earth or moon casts a shadow into each other, resulting in a solar or lunar eclipse.

Models of the Universe

• Ptolemy's model: the Earth is at the center of the universe, and the planets revolve around the Earth in an orbit called epicycle.
• Kepler's model: the Earth and other planets revolve around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
• Kepler's three laws of planetary motion:
  • Law of Ellipse: orbits of all the planets are elliptical with the Sun at one focus of the ellipse.
  • Law of Equal Areas: a line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time.
  • Law of Harmony: the square of a planet's orbital period is proportional to the cube of the semimajor axis of its orbit.

Notable Astronomers

• Tycho Brahe: a Danish astronomer who continuously and precisely recorded the position of the sun, moon, and planets for over 20 years.
• Johannes Kepler: a German mathematician and astronomer who analyzed Tycho Brahe's data and derived the three laws of planetary motion.

This quiz covers the module on models of the universe, from Greek astronomy to Kepler, for Grade 12 physical science students. It assesses their understanding of the historical development of astronomical models.