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The Major Planets
Our Solar System? |
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This section begins with a confession: almost every depiction of the
celestial motion and the positions of celestial bodies presented thus
far has been misleading. (This includes the illustration on the right.)
However, the misconceptions perpetuated are not unique to this website’s
discussion of the basics of celestial motion. These are misconceptions
perpetuated by nearly every astronomy textbook ever written and by every
astronomy professor who has ever drawn on a blackboard, and they
are summed up by the key disclaimer: "not drawn to scale".
In modern astronomy when our solar system is discussed it is typically
characterized in an ordinal fashion in terms of the major planets and their
successively increasing distances from the Sun. The four planets orbiting
closest to the Sun are Mercury, Venus, Earth and Mars, also known as the
terrestrial planets, which are similar in size and are composed primarily of
rock. The four remaining planets, Jupiter, Saturn, Uranus and Neptune are
considerably more massive than the terrestrials and are composed primarily of
gaseous materials, with Uranus and Neptune also being composed of rock and ice.
Between Mars and Jupiter lies the Asteroid belt which is composed of tens of
thousands of rocky celestial bodies or “minor planets” called asteroids which
also orbit the Sun. Rounding out this general characterization of the solar system
is the planet Pluto which was recently demoted from major to minor planet status.
Pluto is often included in a group of lesser known celestial bodies called the
trans-Neptunian objects.
Putting it All on One Page
When these celestial bodies are depicted together it is frequently with their
relative sizes and distances distorted so as to combine all of the celestial
bodies onto a single image or page. The cartoon image in the upper right hand
corner is typical of these convenient characterizations, where the order of the
planets is preserved and their relative distances and sizes greatly diminished to
fit every object into a single picture. The first image below depicts the eight
major planets and the Sun with their relative sizes shown to scale. This image
really demonstrates why Jupiter and Saturn are often called the “gas giants” of
the solar system. (Also shown are the large asteroid Ceres, the minor planet Pluto
and another trans-Neptunian object called Eris.)
The second illustration below depicts the true relative distances from the Sun of each planet
as well as asteroids in the Asteroid belt, measured in astronomical units or (AU)
which is equal to the length of the semi-major axis of the Earth's elliptical orbit or 149,597,870.69 km. Notice that the Earth is positioned at 1 AU and
the farthest major planet Neptune is positioned at 30 AU. Also notice that Pluto's distance
from the Sun varies between 30 and 50 AU, with its orbit sometimes crossing that
of Neptune.
Earth as the Standard Bearer in the Solar System
Reference:
[GSU HyperPhysics]
The solar system scale diagram illustrates an important convention used in modern
astronomy when characterizing celestial bodies in the solar system. This convention uses
the Earth's size, mass and distance from the Sun as a standard unit with which to
characterize every other object. For example, the Earth's equatorial diameter is
12,756 km and its mass is 5.976 x 1024 kilograms, and Venus which is
considered to be the most similar in size to the Earth has a diameter of 12,104 km or
0.949 Earth diameters, and a mass of 4.87 x 1024 kilograms or 0.815 Earth
masses. The table below summarizes a few of the key characteristics of each planet
relative to the Earth's standard distance from the Sun, its diameter and mass.
Apparent or Angular Sizes of Celestial Bodies
One interesting fact that can be culled from these planetary data is the
apparent or angular size of each of the planets as they appear to an observer from Earth.
In the discussion on solar eclipses in the website section on
The Earth and the Moon, the moon's angular size,
which averages about 0.52 degree,
is shown to match closely the angular size of the Sun at 0.53 degree, explaining why the Moon
is able to block the Sun's rays during a solar eclipse. A celestial body's angular
or apparent size is defined in terms of its actual size or diameter and its distance
from the Earth according to
The table below lists the diameter and average distance from the Earth for the Sun,
Moon, the eight major planets and Pluto. Five planets are visible to the naked eye,
Mercury, Venus, Mars, Jupiter and Saturn and are considered the "classical" planets
observable by ancient astronomers. Notice that the apparent size and visiblity
of the planets is dependent on both the planet's size and its distance from the
Earth. For example, although Mars is more than 10 times closer to the Earth than
Jupiter, Mars appears to be only half the size of Jupiter, 0.00695 degree vs. 0.1395
degree, because of the massive size of Jupiter. Similarly, while Saturn is 13 times
further from the Earth than Mercury, the ringed planet's apparent size is greater
than that of Mercury, 0.00574 degree vs. 0.00305 degree.
The next section recommends a number of online resources that can be used for further
study of the relative motion and positions of the major planets as well as other
celestial bodies like asteroids, comets and meteors.
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Basics of Celestial Motion. Copyright 2006
S. E. Scruggs
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