Professor Turing is at the front of the classroom, testing out magically-enhanced diagrams - one for the phases of the Moon, and another of the tides on Earth - in front of the classroom. As he moves the Moon around the images with his wand, the diagrams respond with the appropriate adjustments. Once the class period begins, he looks back at the class and begins his lecture.
Welcome back to Astronomy! Today we will be learning about two additional ways that the Moon affects us magically - phases and tides.
Phases of the Moon: An Explanation
Image Source: NASA
The diagram above explains why the Moon has phases. As you can see, the Earth is the blue sphere near the center of the diagram; the light half represents the side of the Earth that is facing the Sun, and the dark half represents the part of the Earth that is facing away from the Sun. The spheres on the white line/oval represent the Moon in various places as it moves in a counterclockwise orbit around the Earth. Next to the name of the phases are drawings of how the Moon would appear to observers on Earth; the bright portion represents the parts of the Moon that are visible, while the dark portion represents the parts of the Moon that are not visible to observers on Earth. The arrows pointing from the right side of the image to the left? Those represent sunlight, as the Sun is located very far away to the right of the image.
As you can see, only the part of the Moon that faces the Earth and Sun can be seen. Remember why that is the case? Yes, you are correct - this is because, as we mentioned in Year 1, the Sun produces the Solar System’s light and magic, and the planets and moons can only reflect what light and magic is given to them by the Sun.
Draw this diagram in your notes, and then place your quill on the edge of the Earth that is facing the new Moon. Imagine that you are looking at the new Moon. What does the Moon look like from your position? Yes, it looks like you cannot see the Moon because no part of the illuminated side of the Moon is facing the Earth. Move your quill to the position marked “First Quarter”. Notice how you will have a view of a quarter Moon?
Repeat the exercise with the Full Moon, and the Third Quarter; remember to make sure that your quill is on the side of the Earth that is facing the Moon. Now, go back again and see if you can see how all the other phases, including the crescent and gibbous phases can be viewed. I will be walking around the class; feel free to call me over if you need help. The concept on how Moon phases work may be complex at first, but it is easy to understand once you learn the basics, and it is critical for understanding how moonlight and the Moon affect magic on the Earth.
For an explanation of the phases in a different format, here is a Muggle video that explains why phases occur. While the video is optional, as there is no extra content in it that you are required to learn for class, watching the video will help reinforce the information learned in this and other lessons.
Video on Moon Phases (Optional Viewing)
Please note that a common misconception about Lunar phases is that they are created by the shadow of the Earth falling on the Moon. As you can see from the diagram, this is not the case. In fact, the phenomenon that is being described - when the Earth’s shadow hides the Moon - is called a lunar eclipse. We will talk about eclipses next week in class.
The next topic that we will be talking about today are ocean tides. Has anyone here seen tides? What do they look like? Yes, many of you who are from coastal areas or have visited coastal areas have seen tides. At the shores of tidal waters, you can easily see that the water rises and falls throughout the day. If you stay in the same location for a month, you may notice that the water level of the tides changes throughout the month. On some days, the water level may be very high and then immediately become very low. The tide with the biggest change in water level is called a spring tide. On other days, the water level does not change that much. The tide with the smallest change in water level is called a neap tide.
Please note that there are other factors that affect tides. For example, due to the timing of the Moon’s cycle and Earth’s rotation, the timing of the tides will shift. As such, you can use a tidal timetable to help you predict the high and low tides of each day. These charts can also predict the dates and times of spring and neap tides.
Tides are caused mainly by two things - the Sun’s gravity and the Moon’s gravity. As you know, the Earth’s surface is mostly water-covered. While Earth’s gravity holds the water to the Earth, the Sun’s gravity and the Moon’s gravity also have the effect of pulling the water in different directions. While the Sun itself has more gravitational pull in general, the Moon has more of a gravitational pull on the Earth’s oceans than the Sun because the Moon is much closer to the Earth.
Spring and Neap Tides
Image Source: Stony Brook University
As you can see from the diagram above, the spring and neap tides occur at certain times, and these times are related to lunar phases. The top part of the diagram shows a spring tide. A spring tide has the most extreme difference in water levels because the Sun and the Moon pull on the Earth along the same line. Spring tides happen during Full Moons and New Moons. A word that astronomers use to describe three astronomical objects arranged in a straight light is “syzygy” - please remember this word, as you will certainly see it again. In other words, a spring tide occurs during a syzygy or near syzygy of the Sun, Earth, and Moon. The alignment between the Sun, Earth, and Moon also causes magic to be slightly strengthened during a spring tide.
The bottom part of the diagram shows a neap tide. During a neap tide, the Moon and the Sun pull on the oceans with perpendicular gravitational forces - forces that are 90 degrees from each other. Neap tides tend to occur during First and Third Quarter. The arrangement of the Sun, Earth, and Moon causes magic to be slightly weakened during a neap tide.
Thank you for your time. Class is dismissed.