The night sky, and the Earth-Moon-Sun system
(continued)
We measure the sky using angles
We measure the sky using angles
We measure the sky using angles
You can specify the coordinates of an object on the night
sky using two angles
We measure the sky using angles
Full circle = 360°
1° = 60′ (arcminutes)
1′ = 60″ (arcseconds)
Angular size
An object's angular
size appears
smaller if it is
farther away.
We can easily measure an object’s angular position
on the sky, and it’s angular size. But to figure out it’s
true physical size, we also need to know it’s
distance.
Angular size
An object's angular
size appears
smaller if it is
farther away.
•
Q: The Moon is about 0.5° in diameter. What is it’s
approximate physical diameter? Note that the distance
to the Moon, which was first estimated by the ancient
Greeks, is about 2.4x105 miles.
•
Q: The Moon is about 0.5° in diameter. What is it’s
approximate physical diameter? Note that the distance
to the Moon, which was first estimated by the ancient
Greeks, is about 2.4x105 miles.
•
Q: The Moon is about 0.5° in diameter. What is it’s
approximate physical diameter? Note that the distance
to the Moon, which was first estimated by the ancient
Greeks, is about 2.4x105 miles.
•
Q: The Moon is about 0.5° in diameter. What is it’s
approximate physical diameter? Note that the distance
to the Moon, which was first estimated by the ancient
Greeks, is about 2.4x105 miles.
•
Q: The Moon is about 0.5° in diameter. What is it’s
approximate physical diameter? Note that the distance
to the Moon, which was first estimated by the ancient
Greeks, is about 2.4x105 miles.
By coincidence, the Sun and Moon have very nearly the
same angular size
Because of the slight ellipticity of the Moon’s orbit around
the Earth, sometimes the Moon appears slightly larger
than the Sun, and sometimes slightly smaller
•
Q: Approximately what is the physical diameter of the
Sun? One astronomical unit is 9.3x107 miles.
•
Q: Approximately what is the physical diameter of the
Sun? One astronomical unit is 9.3x107 miles.
•
Q: Approximately what is the physical diameter of the
Sun? One astronomical unit is 9.3x107 miles.
•
Q: Approximately what is the physical diameter of the
Sun? One astronomical unit is 9.3x107 miles.
Even thought the Sun and the Moon have close to the
same angular size, the Sun has a physical diameter of
roughly 8x105 mi and the Moon has 2x103 mi. The Sun is
about 400 times larger than the Moon.
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
1ft = 0.305m
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
1ft = 0.305m
0.305m
1=
1ft
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
1ft = 0.305m
0.305m
1=
1ft
m
1 = 0.305
ft
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
1ft = 0.305m
0.305m
1=
1ft
m
1 = 0.305
ft
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
Unit conversions
•
Jon is 7ft tall. How tall is he in meters? Use the fact that
1ft = 0.305m.
•
There is a simple trick: just multiply by 1!
The heliocentric model — Spaceship Earth!
1. How fast is a person standing at the equator moving
around the Earth’s center?
2. How fast is the Earth moving in its orbit around the
Sun?
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
1. How fast is a person standing on the equator moving
around the Earth’s center?
2. How fast is the Earth moving in its orbit around the
Sun?
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The radius of the Earth = 3960 miles
1 astronomical unit = 9.3x107 miles
The Sun is 1.7x109 AU from the center of the galaxy
The Sun orbits the galaxy once in about 2.3x108 years
v=dt; circumference=2𝛑r; 𝛑=3.14159
The heliocentric model — Spaceship Earth!
1. How fast is a person standing on the equator moving
around the Earth’s center?
The heliocentric model — Spaceship Earth!
1. How fast is a person standing on the equator moving
around the Earth’s center?
The heliocentric model — Spaceship Earth!
1. How fast is a person standing on the equator moving
around the Earth’s center?
The heliocentric model — Spaceship Earth!
1. How fast is a person standing on the equator moving
around the Earth’s center?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
2. How fast is the Earth moving in its orbit around the
Sun?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?
The heliocentric model — Spaceship Earth!
3. How fast is the Sun moving in its orbit around the
center of the galaxy?