Definitions

1. Definitions - Heliocentric(geocentric) • Sun centered (Earth centered) • Heliopause • The point at which the solar wind meets the interstellar medium or solar wind from other stars • Heliosphere • The space within the boundary of the heliopause containing the Sun and Solar System

2. Orbital Definitions • Antipodal point • The point that is directly on the opposite side of the planet; e.g., the Earth's north pole is antipodal to its south pole. • Aphelion • The point in its orbit where a planet is farthest from the Sun. • Perihelion • The point in its orbit where a planet is closest to the Sun. • Apoapsis • The point in orbit farthest from the planet. • Periapsis • The point in the orbit closest to the planet. • Apogee • The point in orbit farthest from the Earth. • Perigee • The point in the orbit closest to the Earth.

3. Kepler’s Laws of Planetary Motion 1) The orbit of every planet is an ellipse with the Sun at a focus 2) A line joining a planet and the Sun sweeps out equal areas during equal intervals of time 3) The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

4. Celestial Sphere • Equator – imaginary line bisecting the Earth along a great circle midway between the two poles • North and south poles – antipodal points defined by Earth’s rotational axis

5. Definitions (cont’d) • Celestial Sphere – actually the half sphere of the sky we can see from our vantage point on the Earth’s surface • Zenith – the point directly overhead on the CS • Celestial Meridian – the line bisecting the zenith and passing through both the N and S poles • North/South Celestial Pole – the two antipodal points that have no apparent motion across the CS • North CP is Polaris (the north star) • Due to the Earth’s axial wobble this point wanders over time • Pawnee Indian name meaning “star that does not walk around” • South CP is a point directed to by the long axis of the Southern Cross

6. The Ecliptic Plane • The plane in which the planets of our SS orbit around the Sun • The Sun traces out a closed path on the celestial sphere once each year called the ecliptic • Because the rotation axis of the Earth is tilted by 23.5 degrees with respect to the plane of its orbital motion (which is also called the ecliptic), the path of the Sun on the celestial sphere is a circle tilted by 23.5 degrees with respect to the celestial equator • Important observationally, because the planets, the Sun (by definition), and the Moon are always found near the ecliptic

7. Celestial Sphere and Ecliptic

8. Milky Way Panorama over Racetrack Playa, Nev.

9. Orientation of the Ecliptic Plane and the Galactic Plane

10. Earth’s Orbit around the Sun • Note axial tilt • Note that significant points in orbit are relative to N hemisphere • Earth is actually at aphelion during Summer Solstice and perihelion during Winter Solstice

11. Analemma • The trace of the Suns path during the course of a year • Position of Sun at 12:00 noon • Varies between 23.5 N/S of equator

12. The Motion of the Planets on the Celestial Sphere • All the planets move in nearly circular orbits around the Sun. • Farther planets orbit more slowly • All move in same direction around the sun (counterclockwise, as seen from above northern celestial pole) • Mercury, Venus, Mars, Jupiter, Saturn are visible to the naked eye • Produce no light of their own - all reflected sunlight • Uranus is sometime bright enough to be seen but only ~ magnitude 5.6 at its brightest • Telescopes are always needed to see Pluto and Neptune • Venus and Mercury are closer to Sun than Earth • Do not wander far from Sun in their orbits as seen on the sky • Mercury remains less than 28degrees from the Sun at all times

13. The Motion of the Planets on the Celestial Sphere • Zodiac is formally defined as a band of 18 degrees centered on the ecliptic which marks the wedge around the ecliptic that the planets follow. The band is divided into 12 segments, named for constellations along the ecliptic • A horoscope is technically a map of the position of the Sun, planets and Moon among the constellations of the Zodiac at a particular time

14. The Motion of the Planets on the Celestial Sphere • Solstice • A time at which either day or night is the longest it will be during the year • Occurs around June 21 and December 20 (but not necessarily on those dates) • Called simply summer or winter solstice and mark the beginning of those seasons • Occurs when the sun is directly above 23.5 N latitude (Summer Solstice) or 23.5 S latitude (Winter Solstice) • Will allow one pole to have 24 hours of daylight, while the other pole has a 24 hour night • Equinox • A time at which the days and nights are the same length around the world • Occurs around March 21 and September 21 (but not necessarily on those dates) • Occurs when the Sun is directly over the equator. The vernal equinox marks the beginning of the spring, when the Sun passes northward through the equatorial plane (Mar 21) • The autumnal equinox marks the beginning of fall, when the Sun passes southward through the equatorial plane (Sept 21)

15. 2013 Calendar • Vernal Equinox Mar 20 2013 4:02 AM PDT • Summer Solstice Jun 20 2013 22:04 PM • Autumnal Equinox Sep 22 2013 13:44 PM • Winter Solstice Dec 21 2013 09:11 AM

16. Locations of important points along Earth’s orbit relevant to the ecliptic & equatorial planes

17. The Moon (Luna)

18. Phases of the Moon

19. Tides • Reason for locked lunar orbit • Synchronous rotation • Earth/Moon pair • Decreased over time • Moons orbit has grown over time • Gravitational pull would decrease with increasing distance • Tides have decreased over geologic time • Neap tides • Gravitational pull is minimized • Lower tides • Spring tides • Gravitational pull is maximized • Highest when Sun/Moon are on same side

20. Important Effects • Variation of tidal influences over time • Stabilization of axial tilt • Variable amounts of light during nighttime hours • Slowing of Earth’s rotational speed • Poetical influences