Astronomy Chapter 1

State the period of a sidereal day

23 hours 56 minutes 4.091 seconds

Appreciate that the number of accepted planets has changed over time

1930 pluto was added and deleted in 2006 (Dwarf)

Recall Kepler's first law of planetary motion

1: Law of ellipses The orbits of planets are ellipses, with the Sun at one focus

Recall Kepler's second law of planetary motion

2: Law of equal areas A line drawn from the Sun to a planet sweeps out equal areas in equal intervals of time, as the planet moves in its orbit. (planet must move faster when closer to sun and slowly when further from the sun)

Recall Kepler's third law of planetary motion

3: Law of harmonies Constant = Rcubed over tsquared

Define Ellipse

A regular oval shape, traced by a point moving in a plane so that the sum of its distances from two other points (foci) is constant

State the period of a sidereal month

A sidereal month is 27.322 days, which is the time it takes fro the moon to complete a full orbit around Earth with respect to returning to the same place among the stars.

Define elongation

An angle measured at the Earth between the Sun and a planet.

Recall the configuration of conjunction and opposition for a superior planet

An elongation of 0degrees is conjunction (only one type of conjunction for superior planet) An elongation of 180degrees is called opposition

Explain why opposition is the best time to view a superior planet

At opposition, the superior planet is farthest away from the Sun (180degrees) and also at its closest to Earth and is therefore easier to see. At opposition the planet rises as the sun sets, allowing observation at any time in the night. At opposition the superior will be "full phase" and brightest too.

Explain why celestial bodies (planets) within 11 degrees of the Sun are not visible from Earth's surface.

Due to the Sun's proximity to Earth and the apparent brightness of the Sun, celestial bodies within 11 degrees either side of the Sun are not visible from the Earth's surface.

Describe the general motion of the superior planets, and the reason for retrograde motion

Easterly through the stars. Move westerly during retrograde motion.

Describe the motion of the 7 planets through the Zodiac, close to the eclipitc

Easterly through the stars. The planets of both inferior and superior planetary orbits are close to the ecliptic and are therefore generally seen within one of the constellations of the Zodiac.

Define constellation

Groups of stars that appear as patterns or groupings when viewed from Earth.

Distinguish between inferior and superior planets

Inferior planets are planets whose orbits are within the vicinity of the Sun. Superior planets are planets whose orbits are outside that of Earth's.

Greatest Elongation

Is the maximum value of elongation either East or West of the Sun. This changes depending on the planet being observed.

Superior Conjunction

Occurs when there is an elongation of 0degrees and the planet is positioned on the far side of the sun

Inferior Conjunction

Occurs when there is an elongation of 0degrees and the planet is positioned on the near side of the Sun.

Describe the motion and period of the Sun through the Zodiac, on the ecliptic

The Earth revolves around the Sun every 365.25 days. The apparent path of the Sun as viewed from Earth, the ecliptic, runs through the same constellations every year, the Zodiac.

Define ecliptic

The apparent yearly path of the Sun against the stars.

Conjunction

The lining up of three objects in the sky

Describe the motion of the Moon through the Zodiaz, close to the ecliptic

The moon does not orbit Earth in exactly the same plane as the Sun - its orbit is tilted from the ecliptic by about 5 degrees. It is still within the zodiac.

Define Perihelion

The point in the orbit of a planet/comet/asteroid at which is closest to the sun

Define Aphelion

The point in the orbit of a planet/comet/asteroid at which is farthest from the sun

Explain why greatest elongation is the best time to view an inferior planet

When a planet is at its greatest elongation, it is farthest from the Sun (glare) as viewed from Earth, so its view is also best at that point.

Explain why the length of time from greatest elongation east to greatest elongation west is shorter than the length of time from greatest elongation west to greatest elongation east.

When an inferior planet moves from East to West of the sun, it passes through the inferior conjunction where it is closest to Earth. When a planet moves West to East, it passes through superior conjunction, where it is furthest. Due to the planet's close proximity to Earth when travelling East to West, the planet travels a smaller arc and therefore takes less time.

Describe where (east or west) and when (morning or evening) the inferior planets will be visible in the sky in relation to the position of the Sun.

When an inferior planets is east of the Sun, it is visible as an evening object in the western sky. When an inferior planets is west of the Sun, it is visible as a morning object in the eastern sky.

State the three criteria for defining a planet

a. orbits a star b. is large enough to have settled into a spherical shape c. has cleared the neighbourhood around its orbit of other objects

Distinguish between geocentric and heliocentric model

geocentric = Earth at centre heliocentric = Sun at centre