THE SOLAR SYSTEM
An Overview
Astronomy is the study of the universe. This includes all matter, energy, space, and time.
The concept of the year was developed from observations of the sun's regular cycle of about 365 days.
The concept of the month was developed from observations of the moon's cycle of about 29.5 days.
The solar system consists of the sun, nine planets and their 70 or so moons, asteroids, comets, meteoroids, interplanetary dust particles, gasses and a solar wind composed of charged particles.
Early studies of the solar system incorrectly assumed that the earth was stationary and everything else revolved around the earth. This was the Ptolemy version of the solar system and was called the geocentric (earth-centered)model.
In the early 1500's, Copernicus developed the idea that the sun was the center of the solar system and that the earth revolved around it.
Brahe was a Danish astronomer who made the most accurate measurements at that time of positions of the planets and stars. He did this without a telescope since it had not been invented yet.
Kepler was a German mathematician and astronomer who analyzed the data Brahe collected to develop three laws that govern the motion of planets in the solar system. They are collectively called Kepler's laws of planetary motion.
Kepler's first law states that all planets move in elliptical orbits around the sun with the sun at one focus of the ellipse. An ellipse is shaped like a circle that has been flattened so that it is not perfectly round.
The longest distance across an ellipse is called its major axis and half of this distance is called its semi major axis. The radius of a planet's orbit is approximately equal to its semi major axis.
Distances in the solar system are often expressed in terms of Astronomical Units. 1 AU equals the length of the earth's semi major axis(average distance from the earth to the sun).
Kepler's second law is the law of equal areas. It states that a radius vector drawn from any single planet to the sun will sweep out equal areas in equal times.
This is a result of the conservation of energy. When the planet is far away from the sun, the radius vector forms a long, skinny pie slice because it is moving more slowly(less kinetic, more potential energy). When the planet is close to the sun, the radius vector forms a short, fat pie slice because it is moving faster(more kinetic, less potential energy).
Kepler's third law is known as the harmonic law because of the name of the paper he wrote to describe it(Harmony of the Worlds). It states that the square of the sidereal period of a planet is proportional to the cube of its semi major axis. The equation is:
T2 = kR3
In the equation, T is the sidereal period of the planet(time measured with respect to a distant star), R is the length of the semi major axis of the orbit and k is a proportionality constant. If we express the period in earth years and the radius of the orbit in AU's, k = 1 year2/AU3
Example
The distance from Mars to the sun averages 1.52 AU. Find the time in earth years required for Mars to complete one orbit.
Another very important scientist/astronomer was Galileo. He was the first person to observe the moon and planets through a telescope. He discovered four of Jupiter’s moons which showed that the earth was not the only center of motion in the universe.
Sir Isaac Newton in the late 1600's formulated the laws of gravitational attraction. He used these laws to explain Kepler's laws of planetary motion. He invented calculus to help explain the shape of planetary orbits. His work established the heliocentric theory of the solar system as the correct model.
Planets are classified by location and/or physical description. Planets located inside earth's orbit(Mercury and Venus) are called inferior. Planets located outside earth's orbit are called superior.
Another method classifies Mercury, Venus, Earth, and Mars as terrestrial planets since the chemical and physical properties of the inner planets are relatively similar to those of the earth.
Jupiter, Saturn, Uranus, and Neptune are called Jovian planets since the outer planets have characteristics similar to those of Jupiter.
Pluto is an exception since it does not resemble either the Earth or Jupiter.
If the solar system is viewed from a position far above the Earth's North Pole, the planets all revolve around the sun in a counterclockwise direction. This is called prograde motion and is west to east relative to the sun. All of the planets except Venus and Uranus also rotate west to east so that the sun rises in the east. Venus and Uranus rotate east to west which is called retrograde motion. On these planets the sun rises in the west and sets in the east.
The Jovian planets are much larger in mass and volume than the terrestrial planets. They are sometimes called gas giants.
The solar system is shaped like a disk with a very small bump in the center representing the sun. The planets all orbit the sun on a relatively thin plane with the exception of Pluto. Pluto's orbit takes it well above and below the plane of the solar system.
Time is expressed as either sidereal or synodic. A sidereal period for a planet would be the time between two conjunctions involving the planet and a distant star. This type of time is not significantly affected by the motion of the earth around the sun.
Synodic time is measured relative to the position in the sky of the sun. The revolution of the earth around the sun affects apparent positions relative to the sun.
The sidereal period of the planet Mercury is 88 days. This is the time required for Mercury to complete its orbit around the sun. During this 88 day period, the Earth moves in its orbit so that it takes another 28 days for Mercury to catch back up and create an inferior conjunction with the sun. This is the time difference between the sidereal period and synodic period for Mercury's orbit.
When two objects are in conjunction it means they are on the same meridian in the sky, usually one above the other.
Planets that lie outside of Earth's orbit can reach opposition. This means they are near or on a meridian 180 degrees away from the sun.(Opposite side of the Earth). At opposition a planet is at its nearest to the Earth and is in the sky during night hours.
The Planet Earth
The planet Earth has some properties that are unique.No other planet in the solar system has a large amount of surface water(especially liquid), an atmosphere that contains a large percent oxygen, a temperate climate, and living organisms.
This water and atmosphere contribute to the Earth's large albedo. Albedo is the fraction of incident light that is reflected. The Earth's albedo is .33 and is much larger than the Moon's which is only .07.
The planet Venus has an albedo of .76 which,along with its proximity, makes it the third brightest object in the sky.
It is important to differentiate between rotation and revolution. The Earth revolves around the Sun. Revolution means to move around another object or point.
The Earth rotates about its own axis. Rotation means to spin about some internal axis.
The revolution of the Earth is measured in years. The rotation of the Earth is measured in days.
Because the Earth revolves around the Sun, the Sun follows a path in the sky over a year's time called the ecliptic. If we plot the Sun's position on the celestial sphere at the same time every day we will trace out a circle over a year's time.
This is called the ecliptic plane since eclipses occur when the moon is on or near the ecliptic plane.
Evidence that the Earth rotates is provided by a device called a Foucault pendulum. It consists of a very long low mass wire with a very heavy weight on the end. Once set in motion, it oscillates in the same plane. Over a period of 24 hours the plane of oscillation seems to go through a 360 degree change. This change is due to the rotation of the Earth.
Evidence that the Earth revolves around the Sun comes in the form of stellar parallax and aberration of starlight.
Stars that are nearby seem to change position slightly over a 6 month period. Since the Earth revolves around the Sun, we are about 186,000,000 miles from our initial position and looking at the stars from a slightly different angle. Half of this angle is called the angle of parallax.
Aberration of starlight involves a slight shift in apparent position due to the motion of the Earth. The analogy used here is aberration of raindrops. If you are sitting still in your car in the rain, the raindrops could be coming straight down. As you begin to move forward, they seem to be coming at an angle relative to the vertical. This apparent change in direction due to movement of the observer is called aberration of raindrops or starlight.
The Terrestrial Planets
The terrestrial planets have certain characteristics in common. They are relatively small in size and mass.
They are composed of rocky material and metals.
They are relatively dense.
They have solid surfaces and weak magnetic fields.
Their orbits are close together and close to the Sun.
None has a ring system and only the Earth and Mars have moons(1 and 2).
They rotate slowly from 24 hours to 243 earth days.
Mercury is the closest planet to the Sun. It has a higher orbital speed than any other planet.
Venus is the closest planet to the Earth. It has an atmosphere that is 96% Carbon Dioxide and has clouds composed of sulfuric acid droplets.
Earth has liquid water on its surface, oxygen in its atmosphere and life.
Mars is the planet that most closely resembles the Earth. It has polar caps composed of frozen water and carbon dioxide. It has the largest known volcano in the solar system, Olympus Mons.
The Jovian Planets and Pluto
The characteristics of the Jovian planets are:
They are all much larger than the terrestrial planets.
They are mostly gasseous and have no solid surface.
They have relatively low density.
They have solid cores composed of iron, silicates and ice formed from methane, ammonia, and water
They are farther from the sun than the terrestrial planets.
They experience very low temperatures.
The distances between them are very large.
They have relatively strong magnetic fields.
They have rings and many moons.
They rotate very rapidly.
Jupiter is the largest planet in the solar system. It is nearly large enough to be a star. The great red spot is a characteristic of Jupiter. It is thought to be a like a huge hurricane lasting hundreds of years.
Saturn has easily seen rings. They are composed of ice crystals and ice coated rocks. Saturn's density is so low that it would float in water if we could find an appropriate container and enough water. Saturn's largest moon, Titan, is the only moon known to have an atmosphere.
Uranus has a blue-green color since the methane in its atmosphere absorbs red light. Uranus has an axis of rotation tilted over 80 degrees fron the normal to the ecliptic. Its rotation is retrograde.
Neptunewas discovered through mathematical predictions of its location. Johann G. Galle first observed Neptune using predictions derived from variations in Uranus's orbit. It is a twin of Uranus. It does have a moon, Triton, which orbits Neptune retrograde.
Pluto is the farthest planet from the sun. Some astronomers argue that Pluto is not a planet but that it wandered in from space and was captured by the sun's gravitational field.
The Origin of the Solar System
Several theories have been proposed to explain how the solar system was formed. The currently accepted theory is the condensation theory.
According to this theory, the solar system began as a large, swirling volume of dust and gasses - a rotating primordial nebula.
Gravity caused the matter to be attracted towards the center of the cloud. As this happened, the cloud rotated faster due to conservation of angular momentum.
This rotation caused the cloud to flatten out into a disk shape with most of the matter located near the center.
As the Sun formed in the center, it eventually reached a temperature at which fusion began and it became a star.
The material orbiting the sun coalesced into rings and eventually planets with the exception of the asteroid belt.