NATS1745 6.0 History of Astronomy
Chapter 8 Exam Review
1. What was William Herschel looking for when he discovered Uranus? What
made Herschel particularly suited for discovering planets? In what 2 ways did
Uranus reveal itself to Herschel as a planet? What is it about Uranus' orbit that
caused it to be considered a planet instead of a comet? What is the origin of
• He was actually trying to detect the parallax of stars, because once the stars
parallax could be detected they could calculate a stars distance. He was cataloguing the
coordinates of the stars to look for tiny shifts in a stars position. In the process of tracking
stars he made a discovery of a new planet.
• Herschel was a well-known classical music composer and classical musicians tend to be
good at math and science. While Herschel was reading up between connection of math
and music, his reading led him to astronomy which he became very interested in.
• The two ways are 1. This weird star like thing that he came across looked nebulous (not
exactly point like more like a little blob as opposed to a tiny dot). The star like object
increased when he increased his telescope magnification and it made him realize that it
couldn’t be a normal star, because stars always look point like since they are so far away,
no matter how high we increase our telescope magnification, stars don’t appear to be
getting bigger but this object appeared to be increasing in size with telescope
magnification. 2. He found that the object moved from night to night relative to the stars.
Stars don’t shift so he concluded that he was seeing a comet.
• It was considered as a planet because the object was making a near circular orbit
around the sun (takes 84 years for this planet to make a full circular orbit around the sun
because it's so far from the sun)
• Herschel wanted to the planet after king George, which was refused. Planet was then
named Uranus because it was decided to continue the tradition of the ancient Greeks and
to name the planet after an appropriate god in Greek mythology, so the planet was named
Uranus which is the roman name for the Greek god of the sky and also the father or
Saturn and also the grandfather of Jupiter. Mostly frozen, Known as Gas giant
1. What did Kepler believe existed between the large space between Mars and
Jupiter? Explain why Bode's Law supported this idea.
Kepler observed this large gap between Mars & Jupiter, so he proposed that it contains
an undiscovered planet
• Bode wrote that the orbital radii of the 6 known planet followed a sequence of numbers,
a numeric sequence called Bode's Law. His sequence are very close to the actual
distances of the planets. The sequence suggest that there should be a planet between
mars and Jupiter. When Uranus was found to also follow the sequence a search began an
undiscovered planet at the empty number (space) between Mars and Jupiter
1. What is the name of the object found in 1801 between Mars and Jupiter?
What was this object finally classified as, and why? What is this object a part of?
• Piazzi detected a star-like object in a near-circular orbit around the sun. The little star
like object coordinates changed from night to night so it could not be a star (it wasn’t fixed
and stars are fixed).
• He was a little uneasy about claiming something as momentum as a new planet so he
classified it as a comet, but he did state that it could be something better than a comet.
• Piazzi named the planet ceres. • Ceres was eventually downgraded from planet to asteroid (which means "star-like")
because it was found to be smaller than the moon (or any other planet).
• The object is part of asteroid belt containing hundreds of asteroids between mars and
jupiter, ceres is the largest
1. What caused Adams and Leverrier to suspect the existence of a planet
beyond Uranus? When this planet was found, what was it named, and why?
• Uranus was found to have deviations in its orbit, meaning it doesn’t follow a smooth near
circular orbit around the sun. According to Newtons Laws of Motion, the only thing that
can cause an orbiting body to deviate from its path is another source of gravity, which
means that there must be another object pulling on it and causing it to stray from its orbit
• They did in fact find a planet, in a near circular orbit around the sun at 30AU
• Planet was named Neptune, which is the Roman god of the sea, because of its bluish
1. How were the masses of Uranus and Neptune 1st measured? How do their
masses compare with the other planets? Based on the masses and sizes, what do
we know about their densities and compositions?
• When Triton was discovered to be Neptune's largest moon, its orbit was used to
measure Neptune's mass using Kepler's 3rd Law.
• It was found to be 17 times more massive than the Earth, a little more massive than
Uranus but not as massive as Jupiter and Saturn.
• Its mass divided by its large size tells us that Neptune is a low density planet (another
gas/ice giant) and it has a ring system
• Neptune has 13 known moons (Triton is retrograde so it's not native to the Neptune
1. What is today's theory for why all the planets in our solar system orbit the
Sun in the same direction and in roughly the same plane? What is the observational
evidence for this theory?
• Current theory: After the Sun was born from a spinning collapsing gas cloud, the
remaining gaseous and rocky particles in the Sun's spinning circumstellar disk coalesced
(coalesced: to grow together or into one body) into planets.
• Theory explains that all of the stars (our sun) planets came from the same spinning disk
which means that all of the stars planet should be moving around their star in the same
direction as the spinning disk from which they came, in our solar system is counter
clockwise. Also since the circumstellar disk is flat we would expect that all planets which
formed from the disk would all be confined to the same thin plane of space.
The proof is disks of matter have been found around newborn stars in star-forming
nebulae. Gas clouds were found to contain new born stars with circumstellar disks.
1. What is today's theory for the asteroid belt? What about planetary rings?
• The asteroid belt is likely the leftover debris from the Sun's circumstellar disk that
couldn’t coalesce into a planet due to the opposing gravitational pulls of the Sun and
nearby giant Jupiter. So it's like a failed planet, a planet was supposed to form but it
• Planetary rings are likely the leftover debris from protoplanetary disks which couldn’t
coalesce into a moon. Can be thought of as a failed moon. When each planet formed the
gas and dust near the planet would of gotten trapped by the planets gravity and would of gone into orbit around the planet as a disk of gas and dust particles, disk is called
1. What 3 attributes are unique to the 4 planets beyond Mars? What is the