Astro 120: Fall 2003
The Sky and the Solar System(s)

Review Sheet 1

NOTE: EXAM 1 is October 2 at 6:30pm

### Part I: The Sky, an Introduction - Basic Concepts

The Sky

General scales of the Universe: the astronomical unit, solar system distances and sizes, and the light year.

The Horizon system of coordinates (Altitude/Azimuth). Advantages and disadvantages of such a Local Coordinate system. The concept of the Latitude and Longitude to measure positions on the Earth. The Prime Meridian (Greenwich Meridian) at Longitude =0 degrees. The similar concept of Right Ascension (Celestial Longitude) and Declination (Celestial Latitude). The Celestial North and South poles as extensions of the Earths North and South Poles, The Celestial Equator. The Zero point of Right Ascension = Vernal Equinox. The division of celestial longitude into Hours instead of degrees (24hrs=360 degrees). The fact that Right Ascension INCREASES to the East from the Vernal Equinox. The Autumnal Equinox at RA=12 hrs, Dec=0 degs.

The Sun and its Path on the Sky

The concept of the Ecliptic Plane (the plane containing the orbit of the Earth around the Sun. The tilt of the Earth away from the vertical relative to the Ecliptic (The angle of Obliquity= 23.5 degrees for Earth). The effect of this tilt on the amount of sunlight reaching various part of the Earth at various times of the year. You should know at least four reasons why the its usually much colder in the winter than summer at mid-northern latitudes of the Earth. Be able to calculate the altitude of the Celestial Poles and the highest point on the Celestial Equator from any latitude on the Earth.

The apparent path of the Sun on the Celestial sphere. It passes through the Vernal Equinox moving North (in Spring; March 20/21) at RA=0 Dec=0, reaching Dec=23.5deg and RA=6hrs on June 20/21(Summer Solstice), then back to the Equator (Dec=0) at RA=12hrs on Sept 20/21 and reaching most southerly Dec=-23.5deg at RA=18hrs (Winter Solstice). Understand how this affects its maximum altitude at noon at various times of the year. (Should be able to draw a horizon diagram for the Sun as it transits the local meridian).

The Moon and its Phases

Synodic and Sidereal Month and reason why one is longer than the other. The phases of the Moon. The inverse relationship of the Earth's phases as seen from the Moon. The relationship between the phases of the moon, the time of day and the appearance of the moon in the sky. The fact that the moon always has the same face towards the Earth. The tilt of the Moon's orbit relative to the Ecliptic (Earth-Sun plane). Where is the _____ moon in the sky at ____ time of day?

Path of Moon on Sky

The path of the Moon on the celestial sphere and its relationship to the path of the Sun. The crossing points of the Moon's orbit and the Sun's path on the sky. The significant of the Lunar nodes for the occurrence of Eclipses. The definition of total, partial and annular solar eclipses. Lunar eclipses and their relationship to solar eclipses.

Early Ideas

Ancient Greeks-how they determined the Earth was round and its approximate size. Copernicus and the Sun centered universe. Galileo and his experiments with gravity. Tycho Brahe and Kepler. Kepler's law's of planetary motion. 1st Law of Ellipses, 2nd Law of Areas and third law of Periods (p*p = a*a*a). Concept of eccentricity (e) and semi-major axis of ellipse. Physical picture of an elliptical orbit.

Newton and Gravity and Newton's Laws of Motion

Sir Isaac Newton : Newtonian Concept of Gravity, Gravity as a central force, Newton's 3 laws of motion. The motion of Moon, and Newton's explanation of Kepler's Laws in terms of a theoretical model of gravity.

Orbital mechanics and interplanetary orbits: how to get to Mars via a transfer orbit. Gravity assist to the outer planets.