During the week of Thanksgiving, in the year 2061, an aspiring young astronaut walked along the windy beach with her father. They traversed the sandy coastline, paving a path that other crabs and turtle hatchlings have crossed many times before. As the sun set, she noticed how the water seemed to creep further and further up the shoreline. They continued conversing and laughing along the way, while the waves crashed harmlessly besides them. Eventually, they took a minute to pause and capture the moment. Gazing far into the water, they smiled and said goodnight to the sun. As she watched the daylight disappear before her eyes, Halley started wondering why her feet were now covered with water, when minutes before the sand was dry all around her. She glanced up and asked,
Why is the water creeping up the shoreline?
Her father looked over with a genuine grin. That’s a great question Halley. It’s because the Moon’s gravity has an effect, albeit slight, on Earth.
“Okay—Halley said, but why does that make the water move up the beach and back into the water during the day?”
Well, the Moon’s pull causes there to be two bulges in the earth; one on the side facing the Moon, and the other on the opposite side that faces away from the Moon. The bulges move around the oceans as the Earth rotates, causing high and low tides around the globe.
“Really?” Halley remarked.
Yep, because Earth rotates through two tidal “bulges” every lunar day, coastal areas experience two high and two low tides every 24 hours and 50 minutes.
After letting the answer soak in for a few seconds, they started walking along the beach again. However, Halley’s mind didn’t drift away to another topic, it instead opened a treasure chest of related questions.
“Sooo...what would happen if we didn’t have a moon?”
“For starters, those turtle hatchlings we watch in the summer wouldn’t find their way to the ocean easily, they rely on the light the Moon provides”.
“That wouldn’t be good, what else?”
Amused at his young daughter’s inquiry, he was eager to fuel her curious mind with knowledge on the subject.
“Nights would be much darker than they are now, which other animals also rely on as well all over the world. In fact, a full moon is nearly two thousand times brighter than Venus is at its brightest.
Without the Moon, a day on Earth would only last six to twelve hours. There could be more than a thousand days in one year! That's because the Earth's rotation slows down.
“I can’t even imagine!” she gasped.
Her father went on, “That’s not all, our weather would be pretty wild too. Without the Moon the earth might tilt too far over or hardly tilt at all leading to no seasons or even extreme seasons.
These insights led to questions about the Moon’s beginning.
“Where did the Moon come from?” she pondered.
“That one is up for debate, but the leading theory of the Moon’s origin is that a Mars-sized body (Theia) collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed Moon was in a molten state.” “Within no time at all,” he said with a smirk, “about 100 million years, most of the global magma ocean had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust.”
She grinned, “Dad, you really know how to put things in perspective.”
“Hahaha,” they both chuckled.
More ideas and insights orbited around her head, leading to even more thoughts and questions.
Just a few more she promised, “Why does the Moon always show the same face in the sky and why are the Moon and the Sun the same size?”
“Let’s shed some light on the first one to start. In reality both sides of the Moon see the same amount of sunlight, however only one face of the Moon is ever seen from Earth (about 59% because of lunar libration). This is because the Moon rotates around on its own axis in exactly the same time it takes to orbit the Earth, meaning the same side is always facing the Earth. The side facing away from Earth has only been seen by the human eye from spacecraft.
“Your other question was about the size?” He asked.
“Yes, she clarified, why are they the same size?”
“Ah, well actually they aren’t the same size. The appearance from Earth is a coincidence in life that doesn’t happen often. The sun’s diameter is about 400 times larger than that of the Moon – and the sun is also about 400 times farther from Earth. So the sun and moon appear nearly the same size to us as seen from Earth.
“What are the odds?” She joked.
“Way lower than this tide,” he quipped.
Fulfilled for the moment, she found herself yearning for more space facts about the Moon later that night when they returned home.
She eventually called her friend who was living on our neighboring rock to talk more about the difference in moon phases between a total eclipse of the sun and moon as seen from each other’s home in the solar system.
Halley returned to the topic the next day on Thanksgiving. When it was her turn to share, she cheerfully announced to her family and friends gathered around the table, “I know what I’m adding to the list of what I’m thankful for!”