Definitions

1. Definitions - Heliocentric • Sun 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 (SS)

2. Definitions (cont’d) • Planet - set in 2006 by the IAU • is in orbit around the Sun • has sufficient mass to assume hydrostatic equilibrium (a nearly round shape) • has "cleared the neighborhood" around its orbit • Planetesimal – small irregular bodies within a coalescing protoplanetary disk • Protoplanet – planetary embryo formed through the accretion of planetesimals (Vesta 4 above right • Satellite – a celestial body orbiting a planet (proper noun for ours) • Dwarf Planet • a celestial body orbiting a star that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetismals and is not a satellite (Pluto, Ceres, Eris at right)

3. 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.

4. 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 in Ursa Minor • Greek for Pole Star • Pawnee Indian name meaning “star that does not walk around”

5. 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

6. The Ecliptic Plane • The Sun traces out a closed path on the celestial sphere once each year • This apparent path of the Sun on the celestial sphere is 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 • The ecliptic is important observationally, because the planets, the Sun (by definition), and the Moon are always found near the ecliptic • This is because all of these objects have orbits that lie nearly in the same spatial plane

7. Celestial Sphere and Ecliptic

8. 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

9. 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 • 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

10. 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

11. 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)

12. Locations of important points along Earth’s orbit relevant to the ecliptic plane

13. Orbital Terminology • Orbit – the path defined by a celestial or man-made body in motion around another body • Revolution – one complete trip around the Sun • Ecliptic – the plane of orbit and eccentricity • Eccentricity – the measure of how a planets orbit deviates from a perfect circle • Rotation – length of day or period for a point to complete one turn around an axis • Axis – the imaginary line around which a planet rotates • Axial Tilt – degrees from vertical of a planets axis of rotation • Axial tilt has a great affect on the seasons of a planet

14. Kepler’s 3 Laws of Planetary Motion • 1) The planets orbits are ellipses with the Sun at one major focal point • 2) A line between the Sun and a planet sweeps out equal areas over equal time • 3) The square of the time of one orbit is equal to the cube of its average orbital radius T2 = R3 • i.e. the further a planet is away from the Sun the longer its orbital period is

15. Terminology • Zenith – the point on the celestial sphere directly above the observer • Nadir – the point on the ground directly below an orbiting body or satellite • Azimuth – the horizontal direction on a circle around the observer • Angular Measurements – used to locate or measure the distance between objects on the celestial sphere • Degree – 360 equal divisions of a circle • Arcminute – 1/60 of a degree • Arcsecond – 1/60 of an arcminute • To get a rough estimate of the angular size of objects in space: extending your arm towards the sky your fist covers ~10 degrees your thumb covers ~2 degrees, and your little finger covers ~1 degree. If you look at the Moon, it should take up about 1/2 a degree in the sky