The Night Sky

Long ago, people looked into the night sky and wondered what they were looking at?
How far away are those twinkles in the sky? Could they all be stars, or maybe, could they be
something else? What makes certain lights brighter than others, and how does distance affect
their intensity? These questions and other interesting facts will be reviewed in the following
pages.
One of the most common curiosities regarding the night sky is distance, which can be
very hard to determine. Because space is so vast, scientists must use mathematical methods to
determine how far away, how large, and how bright something actually measures. However,
because of the constantly changing position of Earth and the solar system in relation to the
galaxy, and the incredible distances that separate objects in space, scientists have developed a
different standard unit of measure. The most common unit of measure is a light year. “A light
year is the distance that light can travel in one year” (Giancoli 1000). Using very sophisticated
tools, scientists have measured the speed at which light travels and have found the distance in
one second to equal 3 x 10^8 meters. From this discovery, they have the ability to determine
that one “light minute” equals 18 x 10^9 meters, which calculates one “light year” to equal 9.46
x 10^15 meters (roughly 10^13 kilometers). To promote a better picture of how far ten
trillion kilometers stretches, imagine the distance between the earth and the moon to measure
384,000 kilometers, or 1.28 light seconds; the distance between the earth and the sun is
150,000,000 kilometers, or 8.3 light seconds; and the distance from Earth to the farthest planet,
Pluto, measures 6 billion kilometers or 6 x 10^-4 light years. To envision the almost
unimaginable distances of space, the closest star to the sun, Proxima Centauri, is 4.3 light years
away, which is over 10,000 times farther away than the most distant planet in Earth’s solar
system (Ibid).
Of all the distant objects that are seen in the night sky, the closest objects to Earth are
planets. There are a total of nine planets in the earth’s solar system, including Earth, along with
a few other stellar objects, such as comets, that pass by every so many years (Ridpath 59).
The planets vary greatly in size and composition. Some that shine very brightly can be seen all
year round, while others are very hard to locate and distinguish. The order of the planets in the
solar system goes as follows: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune,
and finally Pluto (Ibid).
Mercury, the smallest planet in the solar system, has a diameter of only 2,900 miles.
This first planet is solid with very little atmosphere and has only six percent the volume of Earth.
Mercury’s mass, however, is not known since there are no satellites, or in other words, moons
orbiting the planet, that would reveal its gravitational influence upon them . If there were moons
orbiting Mercury, then they could help us in determining the planet’s mass by using Einstein’s
Universal Law of Gravitation. However, even without the moons, scientists have been able to
estimate its mass to be about four percent of that of the earth (Pickering 44). Mercury can be
seen in spring and fall, at sunrise or sunset, which is when it is at its greatest distance from the
sun. It can only be seen for a short period of time, though, and is very near the horizon even
when it can be seen (Pickering 46).
The next planet is sometimes known as “the Morning Star” because it is the brightest
object in the night sky. Because of Venus’ rapid orbit of the sun, every 225 days, the time of
year that it can be seen varies from year to year. Venus is so easily visible because it is
incredibly bright compared to normal stars. At its brightest angle, Venus can be up to 4.4 times
brighter than Sirius, the brightest star in the night sky. Venus has a diameter of 7,700 miles, and
is very comparable to the size of Earth. Its mass is 82% of that of the earth’s and its
gravitational pull is 89% the strength of the earth’s (Pickering 47). Even though the proportions
to Earth are so close, Venus has one very distinct difference, and that is its atmosphere. Venus’
atmosphere is so dense that it actually hides the surface of the planet completely and
permanently.