Orbits are the paths followed by objects as they move. Most astronomical objects are in one sort of orbit or another. For example, the Moon orbits the Earth. The Earth orbits the Sun, the Sun orbits the Milky Way, the Milky Way orbits in the Local Group of galaxies and so on.

What determines an object's orbit and what can we learn from it?

An objects' orbit is determined by the forces acting on it. The dominant force is often gravity.

1. What is one property of a body that directly determines its gravitational attraction on another?
A).. Mass
C).. Temperature

Because the gravitational force between two objects depends on their masses and because the force determines the orbit, we can use information about the orbit to deduce the masses. That is, measurements of orbits allow us to "weigh" astronomical objects.

To understand how orbital motion operates we need to look more closely at some general principles of motion. These principles are stated in Newton's Laws of Motion..

Newton's first law of motion states

An object at rest remains at rest and an object in motion moves in a straight line at constant speed unless an unbalanced force acts on it.

In the above we have used the term "unbalanced force." By that we mean that the object may have forces acting on it, but if they act in opposite directions with equal force nothing happens. Only if the forces are not equal and opposite will they set an object at rest in motion or deviate a moving object from a straight line. For example, if you push on shopping cart, you set it moving. However, if a friend pushes equally hard on the opposite side of the cart it doesn't move. In the former case ther force is unbalanced. In the latter it is balanced.

2. A planet orbiting the Sun follows an approximately circular path. Because the path is not a straight line you can conclude
A)...The planet has no mass.
B)... The Sun has no mass.
C)...A force must be acting on the planet.
D)...The forces acting on the planet are in balance.
E)...There is no force acting on the planet.

3. What force is responsible for deflecting a planet's path into a curve around the Sun?
A)... Electricity
B)... Gravity
C)... Magnetism
D)... Inertia
E)... The Coriolis Force

orbit sketch

4. According to Newton's first law, what would happen to the planet in the Figure above if the Force of gravity suddenly shut off?
A)... It would fly outward along arrow B.
B)... It would continue in a straight line along arrow A.
C)... It would continue to move along its curved path.
D)... It would fall into the Sun along arrow C

Sun in Milky Way The figure above shows a sketch of the Milky Way galaxy and the Sun (the yellow dot). The green curve shows the Sun's path (approximately).

5. Why doesn't the Sun follow a straight line path as described by Newton's Second Law?
A)... The Sun would bump into other stars.
B)... A force acts on it.
C)... No force acts on it so it wanders where it wishes.

6. What creates the force that holds the Sun in its orbit?
A)...The Milky Way's spin.
B)... The Milky Way's gravity.
C)... The Milky Way's motion through space.
D)... The Milky Way's magnetic field.
E)... None of the above.

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