What The Moon Does For Earth

It's Tuesday, August eighteenth. My dad texted me this morning about Naga Panchami (NAH-gah pahn-CHA-mee), which is an Indian holiday that falls on the fifth day after the new moon. He was explaining the traditions around it, worshipping the cobra, the connection to coexistence between humans and animals in Hindu culture, and it reminded me that so many of the holidays and traditions I grew up with are timed to the moon. Which made me realize I've never actually thought about what the moon does beyond looking nice at night. I knew it controlled the tides, vaguely, but that was about it. So I looked into it, and the answer is that the moon isn't decorative. Without it, Earth's seasons would be chaotic, our days would be about six hours long, and life as we know it might not exist at all. The thing I've been looking at my whole life and mostly ignoring is one of the main reasons this planet is livable. Earth's axis is tilted at about 23.5 degrees relative to its orbit around the sun, and that tilt is what gives us seasons. When the Northern Hemisphere tilts toward the sun, we get summer. When it tilts away, we get winter. The tilt stays relatively stable, varying by only about one to two degrees over millions of years, and that stability is what allows ecosystems, agriculture, and human civilization to function, because plants and animals can adapt to predictable seasonal patterns.

The moon is what keeps that tilt stable. Its gravitational pull acts on Earth like a counterweight, dampening the wobbles that would otherwise be caused by the gravitational tugging of the sun and Jupiter. Without the moon, Earth's axial tilt could wander dramatically over long timescales, potentially ranging from nearly zero, which would largely eliminate seasons and create a planet with permanent climate zones that never change, to as high as 85 degrees, which would mean periods where the poles point almost directly at the sun while the equator freezes.

Mars is what this looks like in practice. Mars has no large moon, and its axial tilt has varied by more than 25 degrees over millions of years, swinging between periods of relative stability and periods of extreme climate shifts. Uranus is even more dramatic: its tilt is 97 degrees, which means it essentially orbits the sun on its side, producing 42 continuous years of sunlight at one pole followed by 42 continuous years of darkness. Earth's moon prevents our planet from experiencing anything remotely like that, and the predictable seasons that result are so fundamental to how life on Earth works that most scientists consider the moon a necessary condition for the kind of complex life that evolved here. The moon is also the reason you have enough time in a day to do anything useful. When Earth first formed about 4.5 billion years ago, it was spinning much faster than it does now, and a full day lasted roughly six hours. The moon's gravity has been gradually slowing that spin through tidal friction ever since: the moon pulls on Earth's oceans, creating tidal bulges, and the friction between those moving tides and the ocean floor dissipates energy, which over billions of years has transferred rotational momentum from Earth to the moon and slowed our planet's spin from a six-hour day to a 24-hour day.

The process is still happening. Earth's days are getting about 2.3 milliseconds longer per century, which is imperceptible in a human lifetime but adds up over geological time. And the moon itself is slowly drifting away from Earth at a rate of about 3.8 centimeters per year, which scientists have confirmed using laser reflectors that the Apollo astronauts left on the lunar surface in the 1960s and 70s. We know the moon is moving away because we can bounce lasers off mirrors that humans placed there and measure the round-trip time to the millimeter.

The moon also drives the tides, which do a lot more than make the ocean go in and out. Tidal forces mix deep cold water with warm surface water, redistributing nutrients and heat throughout the oceans. Ocean currents driven partly by tidal energy moderate global temperatures and create the climate conditions that support marine ecosystems. Coastal habitats like tidal pools, mangroves, and estuaries depend entirely on the regular rise and fall of water that the moon's gravity produces. The moon doesn't just affect the ocean's surface. It circulates the entire system. The word "month" comes from "moon." That alone tells you how deeply the moon is embedded in how humans organize time. Before clocks, before calendars, before any written system for tracking days, the moon was the most visible and reliable cycle in the sky: roughly 29.5 days from new moon to new moon, visible to everyone, no technology required. It was the first calendar.

The Hindu calendar, which is what my dad was referencing with Naga Panchami, is lunisolar, meaning it uses both the moon's phases and the sun's position to determine dates. The Islamic calendar is purely lunar, which is why Ramadan moves backward through the solar year by about 11 days annually. The Chinese calendar is lunisolar, and Chinese New Year falls on the second new moon after the winter solstice. The Jewish calendar is lunisolar, and holidays like Passover and Rosh Hashanah are set by the moon's cycle. Even Easter, which most Western Christians think of as a fixed spring holiday, is set by the moon: it falls on the first Sunday after the first full moon after the spring equinox, which is why the date moves every year and most people couldn't explain why if you asked them.

Billions of people alive today schedule their most important religious and cultural events around the same lunar cycles that humans have been watching for tens of thousands of years. My dad's text about a holiday timed to the fifth day after the new moon is part of a tradition that connects directly to the earliest human attempts to make sense of time, and it works because the moon's cycle is the one natural rhythm that's visible everywhere on Earth, to everyone, every single month, without any technology or infrastructure required. We built our sense of time around the moon long before we understood that the moon was also the reason we had stable seasons and 24-hour days to organize in the first place. My dad texted me about a holiday and I ended up learning that the moon is the reason Earth has predictable seasons, the reason our days are 24 hours instead of six, and the clock that billions of people still use to schedule their most important days. It's easy to look up at it and think it's just there, but almost nothing about life on this planet would work the way it does without it.

Stay informed, stay curious, and we'll see you tomorrow.