Astronomy – Distance in The Stars and The Solar System
***All italic words are located at the bottom accompanied by their definitions***
The Solar System
•The solar system consists of the Sun, nine planets, sixty-eight satellites (of the planets), large numbers of comets and asteroids, and the interplanetary medium.
•All the planets orbit in the same counter-clockwise direction looking down from above the Sun’s north pole
or in a prograde direction. All except for Venus and Uranus, whose rotations are defined as retrograde.
•The nine planets are classified in several ways:
By composition:
•Terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:
•The terrestrial planets are composed mostly of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and very few satellites.
o Jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:
? The gas planets are composed primarily of hydrogen and helium, and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.
By size:
o Small planets: Mercury, Venus, Earth, Mars and Pluto.
? The small planets have diameters less than 13000 km.
o Giant planets: Jupiter, Saturn, Uranus and Neptune.
? The giant planets have diameters greater than 48000 km.
o Mercury and Pluto are sometimes referred to as lesser planets (not to be confused with minor planets which is the official term for asteroids).
o The giant planets are sometimes also referred to as gas giants.
By position relative to the Sun:
o Inner planets: Mercury, Venus, Earth and Mars.
o Outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto.
o The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.
By position relative to Earth:
o Inferior planets: Mercury and Venus.
? They are closer to the Sun compared to the Earth.
? The inferior planets show phases like the Moon’s when viewed from Earth.
o superior planets: Mars thru Pluto.
? farther from the Sun than Earth.
? The superior planets always appear full or nearly so.
By History:
o classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
? known since pre-historical times
? visible to the unaided eye
o modern planets: Uranus, Neptune, Pluto.
? discovered in modern times
? visible only with telescopes
Stars:
• Are huge spherical masses of gas that began radiating energy because of nuclear reactions occurring deep within their interiors
• They differ in size, shape, luminosity, temperature, and composition
Distance To Stars:
First measured by Friedrich Bessel in 1838, method called Trigonometric Parallax
• As the earth moves around the sun, a star relatively nearby will appear to shift its position in the sky relative to more distant stars. By measuring these minute angular shifts, the distance of a star can be determined trigonometrically. Half of the total apparent shift of the star with respect to the background is called the star’s trigonometric parallax. It follows that the parallax of a star is the angle formed t the star by the triangle formed between the star and each end of the radius of the earth’s orbit.
Glossary:
Jovian planets: The planets with orbits beyond that of Mars: Jupiter, Saturn, Uranus, Neptune, and Pluto.
Parallax: The apparent change in position of a star relative to the distant, background stars as seen from the Earth at opposite points in its orbit around the sun.
Prograde: Counterclockwise, when viewed from north of the ecliptic, rotation of a celestial body around its axis.
Retrograde motion: The motion of a planet from west to east among the stars over a lengthy period of time. It is when the earth overtakes another planet as they both orbit the sun.
Star: A celestial body made up of hot gases held together by its own gravity and emitting light and heat resulting from its internal nuclear reactions. Our Sun is a typical star.
Terrestrial planets: The planets with orbits within the orbit of Jupiter, Mercury, Venus, Earth, and Mars.
The Distance To A Star
Tan( 1 ) = 93 000 000 Miles
9000 X
X= 4.8 x 10
A.U.= Astronomical Unit= 150 million Kilometres = 93 million Miles
1 minute= 1 1 second= 1
60 3600
A= 0.4 seconds = 1
9000