Unit 1 - Science Literacy & Space
1.2 - Space
Day 3 - Due 9/26(A) & 9/27(B)
Earth as a Planetary Body
Earth is an inner planet in the solar system and it is very much like the other inner planets, at least in its size, shape, and composition. But many features make Earth very different from the planets and any other planet that we know of so far.
Earth’s Shape
Earth is a sphere or, more correctly, an oblate spheroid, which is a sphere that is a bit squished down at the poles and bulges a bit at the equator. Or to be more technical, the minor axis (the diameter through the poles) is smaller than the major axis (the diameter through the equator). Half of the sphere is a hemisphere. North of the equator is the northern hemisphere and south of the equator is the southern hemisphere. Eastern and western hemispheres are also designated.
What evidence is there that Earth is spherical? What evidence was there before spaceships and satellites?
Try to design an experiment involving a ship and the ocean to show Earth is round. If you are standing on the shore and a ship is going out to sea, the ship gets smaller as it moves further away from you but the ship’s bottom also starts to disappear as the vessel goes around the arc of the planet (Figure below). There are many other ways that early scientists and mariners knew that Earth was not flat.
Earth is an inner planet in the solar system and it is very much like the other inner planets, at least in its size, shape, and composition. But many features make Earth very different from the planets and any other planet that we know of so far.
Earth’s Shape
Earth is a sphere or, more correctly, an oblate spheroid, which is a sphere that is a bit squished down at the poles and bulges a bit at the equator. Or to be more technical, the minor axis (the diameter through the poles) is smaller than the major axis (the diameter through the equator). Half of the sphere is a hemisphere. North of the equator is the northern hemisphere and south of the equator is the southern hemisphere. Eastern and western hemispheres are also designated.
What evidence is there that Earth is spherical? What evidence was there before spaceships and satellites?
Try to design an experiment involving a ship and the ocean to show Earth is round. If you are standing on the shore and a ship is going out to sea, the ship gets smaller as it moves further away from you but the ship’s bottom also starts to disappear as the vessel goes around the arc of the planet (Figure below). There are many other ways that early scientists and mariners knew that Earth was not flat.
Even the ancient Greeks knew that Earth was round by observing the arc shape of the shadow on the Moon during a lunar eclipse.
The Sun and the other planets of the solar system are also spherical. Larger satellites, those that have enough mass for their gravitational attraction to have made them round, are as well.
Earth’s Motions
Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earth’s rotation. At the same time that the Earth spins on its axis, it also orbits, or revolves around the Sun. This movement is called revolution.
An observer in space will see that Earth requires 23 hours, 56 minutes, and 4 seconds to make one complete rotation on its axis. But because Earth moves around the Sun at the same time that it is rotating, the planet must turn just a little bit more to reach the same place relative to the Sun. Hence the length of a day on Earth is actually 24 hours. At the equator, the Earth rotates at a speed of about 1,700 km per hour, but at the poles the movement speed is nearly nothing.
Earth’s Revolution
For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. The gravitational pull of the Sun keeps Earth and the other planets in orbit around the star. Like the other planets, Earth’s orbital path is an ellipse (Figure below) so the planet is sometimes farther away from the Sun than at other times. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd and the furthest is at aphelion (152 million km) on July 4th. Earth’s elliptical orbit has nothing to do with Earth's seasons.
The Sun and the other planets of the solar system are also spherical. Larger satellites, those that have enough mass for their gravitational attraction to have made them round, are as well.
Earth’s Motions
Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an axis. Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earth’s rotation. At the same time that the Earth spins on its axis, it also orbits, or revolves around the Sun. This movement is called revolution.
An observer in space will see that Earth requires 23 hours, 56 minutes, and 4 seconds to make one complete rotation on its axis. But because Earth moves around the Sun at the same time that it is rotating, the planet must turn just a little bit more to reach the same place relative to the Sun. Hence the length of a day on Earth is actually 24 hours. At the equator, the Earth rotates at a speed of about 1,700 km per hour, but at the poles the movement speed is nearly nothing.
Earth’s Revolution
For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. The gravitational pull of the Sun keeps Earth and the other planets in orbit around the star. Like the other planets, Earth’s orbital path is an ellipse (Figure below) so the planet is sometimes farther away from the Sun than at other times. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd and the furthest is at aphelion (152 million km) on July 4th. Earth’s elliptical orbit has nothing to do with Earth's seasons.
During one revolution around the Sun, Earth travels at an average distance of about 150 million km. Earth revolves around the Sun at an average speed of about 27 km (17 mi) per second, but the speed is not constant. The planet moves slower when it is at aphelion and faster when it is at perihelion.
The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2o on its axis. During the Northern Hemisphere summer the North Pole points toward the Sun, and in the Northern Hemisphere winter the North Pole is tilted away from the Sun (Figure below).
The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2o on its axis. During the Northern Hemisphere summer the North Pole points toward the Sun, and in the Northern Hemisphere winter the North Pole is tilted away from the Sun (Figure below).
You've heard that Earth revolves around the sun. Well, that's not quite true! Here's what's really going on:
The exact center of all the material (that is, mass) that makes up an object—whether a planet or a pencil—is called its "center of gravity." For example, if you have a straight stick, like a ruler, there's a place at the middle where you can balance it on your finger. That's its center of gravity.
The exact center of all the material (that is, mass) that makes up an object—whether a planet or a pencil—is called its "center of gravity." For example, if you have a straight stick, like a ruler, there's a place at the middle where you can balance it on your finger. That's its center of gravity.
But the center of gravity may not be the point that looks like the middle of the object. Some parts of the object may be heavier (denser) than others. A sledge hammer is heavier on one end than the other. Its center of gravity is much closer to the heavy end than the lighter end.
To get an idea of where the center of gravity is, rest the ends of any object like the ruler or a pencil on one finger from each hand. Slowly move your fingers together without dropping the object. Your fingers will meet underneath the object's center of gravity. You can balance the object on one finger at that special place.
The actual center of gravity could be close to the surface if, for example, the object is flat like a ruler or a dinner plate. Or the center of gravity could be deep inside if the object is "three-dimensional," like a box or a ball. And if you let the object spin (like when you throw it), it will try to spin about that point.
What's a barycenter?
In the case of the Earth and the Sun, both bodies orbit around the very center of the mass (similar to center of gravity) between them. This point is called the "barycenter."
Earth and the Sun are "connected" by the gravity pulling them together. It's just like the light end and heavy end of the sledge hammer. Compared to the size of the Sun, though, Earth is about like a flea on a cat! So the center of mass between the Earth and the Sun is almost—but not quite—the very center of the Sun.
Jupiter, on the other hand, is 318 times as massive as Earth. Therefore, the barycenter of Jupiter and the Sun is a bit further from the Sun's center. So, as Jupiter revolves around the Sun, the Sun itself is actually revolving around this slightly off-center point, located just outside its surface.
Thus, a planet the size of Jupiter will make its star wobble a tiny bit. This picture shows you that the center of mass of a star and the barycenter of a star and a planet can be slightly different points
The barycenter "wobble" gives us a way to find planets around other stars.
The actual center of gravity could be close to the surface if, for example, the object is flat like a ruler or a dinner plate. Or the center of gravity could be deep inside if the object is "three-dimensional," like a box or a ball. And if you let the object spin (like when you throw it), it will try to spin about that point.
What's a barycenter?
In the case of the Earth and the Sun, both bodies orbit around the very center of the mass (similar to center of gravity) between them. This point is called the "barycenter."
Earth and the Sun are "connected" by the gravity pulling them together. It's just like the light end and heavy end of the sledge hammer. Compared to the size of the Sun, though, Earth is about like a flea on a cat! So the center of mass between the Earth and the Sun is almost—but not quite—the very center of the Sun.
Jupiter, on the other hand, is 318 times as massive as Earth. Therefore, the barycenter of Jupiter and the Sun is a bit further from the Sun's center. So, as Jupiter revolves around the Sun, the Sun itself is actually revolving around this slightly off-center point, located just outside its surface.
Thus, a planet the size of Jupiter will make its star wobble a tiny bit. This picture shows you that the center of mass of a star and the barycenter of a star and a planet can be slightly different points
The barycenter "wobble" gives us a way to find planets around other stars.
We can take advantage of this bit of knowledge and look for large planets in other solar systems by learning to detect this type of tiny wobble in the star's position.
As seen from above, a large planet orbits a star–or rather the star and planet orbit their shared center of mass, or barycenter.
As seen from above, a large planet orbits a star–or rather the star and planet orbit their shared center of mass, or barycenter.
As seen from the side, a large planet and star orbit their shared center of mass, or barycenter, with the star seeming to shift back and forth.
Questions
1. When you watch a tall ship sail over the horizon of the Earth, you see the bottom part of it disappear faster than the top part. Why does this happen?
2. Describe the difference between Earth’s rotation and its revolution.
3. What is the force that keeps the Earth and other planets in their orbital paths?
4. In its elliptical orbit around the Sun, the Earth is closest to the Sun in January. If Earth is closes to the Sun in January, why is January winter in the Northern Hemisphere?
5. What is the barycenter?
1. When you watch a tall ship sail over the horizon of the Earth, you see the bottom part of it disappear faster than the top part. Why does this happen?
2. Describe the difference between Earth’s rotation and its revolution.
3. What is the force that keeps the Earth and other planets in their orbital paths?
4. In its elliptical orbit around the Sun, the Earth is closest to the Sun in January. If Earth is closes to the Sun in January, why is January winter in the Northern Hemisphere?
5. What is the barycenter?