Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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phases of Moon

Sun always illuminates one half of Moon. However, as Moon orbits Earth, we can only see a portion of the illuminated side of Moon. The portion that we see depends on the location of Moon relative to Sun.

View this APOD (Astronomy Picture of the Day) by Antonio Cidadao to see an animated sequence of images of Moon during one lunar month, or lunation. A copy of the picture is shown below. But be sure to follow the link to the web site so that you can read more about it.


Study the simulation shown below. (Written by Mario Belloni and Todd Timberlake.) There is a lot of information in this simulation that will help you understand why we see phases of Moon.

  • The green dot is YOU standing on Earth. When are in the green light cone, you see Moon. When you are not in the green light cone, you can not see Moon.
  • When you are in the blue part of Earth, it is daytime. When you are in the black part of Earth, it is nighttime.
  • Sun always illuminates half of Moon, the half facing Sun.
  • By understanding the picture, you can predict what the phase of Moon will be if given the orientation of Earth, Moon, and Sun.
  • Sunrise occurs as YOU enter the blue part of Earth.
  • Sunset occurs as YOU exit the blue part of Earth.
  • Moonrise occurs as YOU enter the green light cone.
  • Moonset occurs as YOU exit the green light cone.
  • Noon is when you are directly facing Sun.
  • Midnight is when you are directly facing away from Sun.

 

 

You can perhaps understand this better if you demonstrate it, as described in the lab activity below.

Lab Activity


In this lab, you will view the phases of Moon using a ball and lamp.

 

Materials Needed

You will need a white styrofoam ball (the larger the radius, the better) and lamp (remove the lampshade).

Setup

  1. Go to a hobby or craft shop and purchase a large styrofoam ball with a radius of at least 2.5 inches.
  2. Obtain a lamp and remove the lampshade. If you have a 100 W light bulb, insert it into the lamp. Otherwise, use the brightest bulb that you have.
  3. Place the lamp at the center of a room. Turn the other lights off.

Procedure

  1. Face the lamp from about 4-6 ft away.
  2. Hold the ball above your head and directly in front of you, between you and the lamp. Look at the ball. Notice that it is not directly illuminated. Sure, some light reflects off of walls and indirectly illuminates the ball. However, there is no direct light from the lamp that illuminates the side of the ball that you are looking at. In fact, if this were space, you couldn't see the ball at all.
  3. While holding the ball in front of you and above your head, slowly turn (in place) to the left and watch the ball. Notice that as you rotate 1/8 of a turn, you'll notice a waxing crescent with the right side of the ball direclty illuminated.
  4. After 1/4 of a turn, you'll notice the first quarter with the entire right side of the ball directly illuminated.
  5. After 1/2 of a turn, you'll notice the "full Moon." The entire face of the ball that is facing you is directly illuminated. You are directly between the ball and the lamp with the ball in front of you and Sun directly behind you. This is local midnight.
  6. Turn 1/8 of a rotation further, and you'll see a waning gibbous.
  7. Turn to the point where the entire left side of the ball is directly illuminated, and this is the third quarter.
  8. Turn another 1/8 of a rotation further, and you'll see a waning crescent with just a sliver on the left side illuminated.
  9. Finally, you end up facing the lamp again with Moon between you and the lamp. This is local noon. Moon is in the sky but you can't see it because the part of Moon facing you is not directly illuminated.

You should know the names of various phases of Moon (see textbook) and be able to identify the phase of Moon based on the orientation of Earth, Sun, and Moon. Also, based on the orientation of Sun, Earth, and Moon at any point, you should know approximately what time it is if Moon were highest in the sky (i.e. you are facing Moon in the animation).

 

 

 

 

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