Answered by Dr. Brian Tonks, Astronomy Professor, BYU-Idaho
You can see the Moon during the day because it slowly orbits the Earth. As a result, the Moon lies opposite the Sun during part of its orbit and we see it all night long. During part of its orbit, it lies near the Sun and we see it during the daytime, if we see it at all. As it orbits the Earth, the Moon also goes through its series of “phases”. Sometimes we see the entire Moon (we call this a “full” moon). Sometimes we see half of the Moon. Sometimes the Moon appears as a crescent. It takes the Moon approximately 1 month to orbit the Earth and go through its cycle of phases. Let me use diagrams to help you better understand how it works.
Figure 1 shows an observer standing on Earth (the person on this Earth is NOT drawn to the correct scale—he/she would be a super-tall giant!). In this picture, we are in the Starship Enterprise looking down on the Earth’s North Pole. From this position, we see the Earth rotating in a counterclockwise direction once every 23 hours and 56 minutes. We also notice that the Earth slowly orbits the Sun, also in a counterclockwise direction. The Sun’s rays in the diagram show that the Sun is far away, off the left side of the figure. The Moon also orbits the Earth in a counterclockwise direction. The Earth carries the observer as it rotates. When the person stands at position a in the diagram, he/she sees the Sun crossing a circle that connects straight north and straight south (the Sun will be highest in the sky at this time). We call the time when the Sun reaches this North-South circle “noon”. Notice that the Sun lights up half of the Earth and that the other half of the Earth is dark. I’m sure you know that it’s daytime when the Sun is visible and nighttime when we can’t see it.
Because Earth rotates counterclockwise, after about 6 hours the person will have moved to position b in the diagram. Notice that the Sun will appear low in the person’s sky (approximately sunset). The person’s north-south circle is downward in this figure. Six hours later, the person rotates to position c. Because the Sun lies on the opposite side of the Earth, it is now midnight and the person’s sky is on the right side of the figure.
Continued next week: Let’s add the Moon
Earth’s Orbit around the Sun
The Earth’s path around the Sun is called its orbit. Let‘s model the Earth’s orbit.
Materials: 3 inch ball (Earth), lamp without a shade (Sun), 200-watt bulb, extension cord, bamboo stick
- Find a space about 10’ x 10’ to provide ample room for the demonstration.
- Place the lamp (Sun) in the center of the space. Plug it in and turn it on. The Earth spins counter clockwise around the Sun. The Earth is much smaller than the Sun and is very far from the Sun (93,000,000+ miles). The model we are making is not to scale.
- Place the 3” ball on a bamboo skewer to represent Earth. Hold the skewer vertically so that the Earth can be held from above and rotated.
- Start by walking counterclockwise in a circle around the Sun approximately 4 feet away from the Sun lamp in the center. This motion represents the Earth’s orbit around the Sun. How long does it take for the Earth to make a complete revolution (orbit) around the Sun? (365.25 days)
- Make another orbit around the Sun while turning the skewer counterclockwise to demonstrate the Earth’s spin. How long does it take for the Earth to make a complete rotation (spin)? (24 hours)
Did You Know? It takes the Earth approximately 365 days, 5 hours, 48 minutes, and 45 seconds to orbit the Sun. Leap years are needed to keep our modern day calendar in alignment with the Earth’s revolutions around the sun.