This document provides an overview of planetary motion and related concepts in astronomy. It describes the hierarchy of structures in the universe from galaxies to solar systems. Key points include: galaxies contain hundreds of billions of stars; a light year is the distance light travels in one year; stars are balls of hydrogen and helium that shine through nuclear fusion; solar systems contain planets and other objects orbiting a star; and planetary orbits follow Kepler's laws, such as elliptical orbits where planets sweep equal areas in equal times.

1. PLANETARY MOTION
 Module 15
Photo Credit: NASA

2. The universe is defined as all matter and
energy. It is known that the universe is
expanding and that all other galaxies are
rushing away from us. The universe is made
up of many different structures arranged in a
fairly well-defined hierarchy.
Photo Credit: NASA

3.  Galaxies-are large groups of stars, nebulae,
and other celestial objects. There are
hundreds of billions of galaxies.
Photo Credit: Public Domain Images

4.  A light year is the time it takes light to
travel in one year, 186,000 miles per
second.
Photo Credit: Britannica.com

5. Galaxies are separated by hundreds,
thousands or millions of light years.
Photo Credit: NASA

6. Stars are huge balls of hydrogen and helium
held together by gravity. There are hundreds
of billions of stars contained in each galaxy.
Photo Credit: NASA

7. Stars shine because they are producing energy through
nuclear fusion. This happens when two atoms of
hydrogen combine to form a helium atom and a lot of
energy is released in the process.
Photo Credit: NASA/SDO, AIA

8. A solar system consists of a star and all the celestial
objects that orbit that star. Those objects can
include: planets, moons, comets and asteroids.
Photo Credit: NASA

9. Our solar system contains a star, (we
call it sun) and 8 planets with many
other objects orbiting the sun.
Photo Credit: NASA

10. Many of the hundreds of billions of stars
in our Milky Way galaxy and other
galaxies have planets orbiting them.

11. Planets- The International Astronomy Union has an
official definition for a planet. It states, "A 'planet' is
defined as a celestial body that (a) is in orbit around
the sun.
Photo Credit: NASA

12. (b). Has sufficient mass for its self-gravity to
overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly
round) shape.
Photo Credit: Planetfacts.org

13. And (c) it has cleared the neighborhood
around its orbit."
Photo Credit: bbc.co.uk

14. Satellites are objects that revolve around objects of
greater mass. They can be man-made or natural like
our moon. Earth is a satellite of the sun and the sun is
a satellite in the Milky Way.
Photo Credit: Earthsky.org

15. Ancient astronomers believed the Earth was
the center of the Universe. By 1543
Copernicus had figured out the motions of
the planets made if they moved around the
Sun. This is the Heliocentric Model.
Photo Credit: fourmilab.ch

16. The Earth was one of them and if the more distant
ones moved more slowly, so sometimes the Earth
overtakes them, and then they seem to move
backwards for a while. The orbits of Venus and
Mercury were inside that of the Earth, so they
never move far from the sun.
Venus Crossing the Sun. Photo Credit: NASA

17. Planetary orbits according to Kepler’s Laws:
Johannes Kepler (1571- 1630) worked as
an assistant Tycho Brahe in an
observatory near Copenhagen. Brahe spent
his life trying to find the proof of the
heliocentric model (sun is in the center of
the solar system) by studying the orbit of
Mars. After his death, Kepler used
Brahe’s data and mathematics to form the
3 laws of planetary orbits.

18. Johannes Kepler
Photo Credit: etsu.edu
Tycho Brahe
Photo Credit: Famousscientists.org

19. First Law- Planets move around the sun in
ellipses. Ellipses are not circles but more oval
shaped. Also, most of the planets are in or
around the same plane. You can think of this
as a big pancake with all the planets orbiting
the sun in ellipses.
Photo Credit: http://astronomy.swin.edu.au

20. An ellipse happens because there are 2 points
called the foci, with the sun being one of the
foci and usually nothing in the other foci.
Photo Credit: askamathematician.com

21. Kepler’s Second Law- The line connecting
the Sun to a planet sweeps equal areas in
equal time.
Photo Credit: le.ac.uk

22. Although the orbit is symmetric, the
motion is not. A planet speeds up as it
approaches the sun, gets its greatest
velocity when passing closest, and then
slows down again.
Photo Credit: zebu.uoregon.edu

23. What happens is best understood in terms of
energy. As the planet moves away from the sun, it
loses energy by overcoming the pull of gravity, and
it slows down, like a stone thrown upwards. And like
the stone, it regains its energy as it comes back. (Dr.
David Stern, an overview for science teachers.)
Photo Credit: mrfizix.com

24. Kepler’s Third Law- “The Law of Harmonies”.
This is the law that Kepler used to measure
the distance of planets from the sun. He
came up with a mathematical formula that is
still used today.
Photo Credit: NASA

25. The Moon orbits around the Earth. Since its size
only changes slightly, its distance stays about the
same, and hence its orbit must be close to a circle.
To keep the Moon moving in that circle--rather than
wandering off--the Earth must exert a pull on the
Moon, and Newton named that pulling force
gravity.
Photo Credit: newgrounds.com

26. Motions of the Earth- The two main motions of Earth
are rotation and revolution.
Rotation is the spinning of Earth on its axis and the
cause of night and day. Each rotation equals 24
hours, and since the Earth is roughly 24,000 miles in
circumference, that means we are spinning about
1,000 miles per hour.
Photo Credit: lpi.usra.edu

27. Revolution is the period of time it takes for the
Earth to orbit the sun at a speed of 66,000 miles per
hour, in one year. It takes Earth 365.25 days to
make one complete orbit. That leaves one extra
day every four years that we call leap day or year.
Photo Credit: srh.noasrh.noaa.gova.gov

28. Seasons- Earth’s axis rotation is tilted
about 23.5 degrees, this is the reason for
the seasons.
Photo Credit: csep10.phys.utk.edu

29. Precession- This is the change in direction of
the axis, but without any change in the tilt.
This changes the relative positions of the stars
but does not affect the seasons.
Photo Credit: earthobservatory.nasa.gov

30. Nutation is a wobbling around the precessional axis.
This change in the angle---half degree one way or the
other. This occurs over an 18 year period and is due to
the Moon exclusively. This would very slightly
increase or decrease the amount of seasonal effects.
Photo Credit: www2.jpl.nasa.gov

31. Barycenter is the point between two objects where
they balance each other. (For example, it is the
center of mass where two or more celestial bodies
orbit each other. )
Photo credit: spaceplace.nasa.gov

32. When a moon orbits a planet, or a planet orbits a
star, both bodies are actually orbiting around a
point that lies outside the center of the primary (the
larger body).
Photo Credit: lpl.arizona.edu

33. For example, the moon does not orbit the exact
center of the Earth, but a point on a line between the
Earth and the moon approximately 1,710 km or 1062
miles below the surface of the Earth, where their
respective masses balance.
• This is the point about which the Earth and moon
orbit as they travel around the sun.
Photo Credit: cde.nwc.edu

34. Motions of the sun- Our star (called sun) is not
stationary in our solar system. It actually moves as
the planets tug on it, causing it to orbit the solar
system’s barycenter. The sun never strays too far
from the solar system barycenter.
Photo Credit: lcogt.net

35. Tides- What kind of force can move Earth’s
oceans back and forth daily? Gravity! It is
the gravity exerted on the Earth by the
moon and to a lesser extent, by the sun.
Photo Credit: pbs.org

36. Photo Credit: onegeology.org
Even though the enormity of the sun should produce
more gravity, the sun is only responsible for 46
percent of the gravitational pull on the oceans. This is
because the sun is so far away.

37. Circumference- The Earth like most celestial
bodies is spherical because gravity is pulling
the matter in from all directions.
Photo Credit: fstdt.com

38. Photo Credit: NASA
However, the circumference around the equator is
slightly bigger than the circumference around the
poles.
The reason for this is that the Earth is rotating on its
axis causing it to bulge out at the equator.