If asked, most of us will agree that the earth moves in two ways:
1. On its axis (axis is an imaginary line that goes through the North and South Poles).
2. Around the sun.
But before we can say that, we need to raise the question: how do we even measure this movement? If we need to measure anything’s true movement, we need to have a relative point. So depending on where we observe the movement from, Earth moves differently.
To understand this better, think if you were in a bus and you are moving in a circle inside the bus. Depending on how someone looks, you are moving in two directions. If someone is inside the bus, he will see that you are moving in a circle, and if someone from outside the bus looks, then you are actually moving in a straight line for him.
On Earth’s Axis
If we apply the same understanding to our Earth’s rotation, then we can look from a faraway distant star and see how Earth actually moves. This is called a sidereal day (23 h 56 min 4.0905 s), which means it is the amount of time it takes for the Earth to rotate on its axis once relative to the stars (used especially by astronomers).
On the other hand, a solar day is the amount of time it takes for the Earth to rotate on its axis once relative to the Sun (24 hours long on average). But this gets even more complicated as we go deeper because if we now think about the Earth’s movement, then we can see that the Earth is tilted on its axis. Earth is tilted at an angle of 23.5 degrees away from vertical, perpendicular to the plane of our planet’s orbit around the sun. Earth moves in a counterclockwise direction at about 1,000 miles per hour (460 m/s or 1,600 km/hr).
What Makes the Seasons?
If we take the Earth’s rotation around the sun into account, then we can see things more clearly. As the Earth moves around the sun, Earth also moves on its axis with a tilted angle of 23.5 degrees away from the vertical, which causes the seasons like summer and winter and also why it can be summer in one country and winter in another at the same time. Not only that, our days aren’t perfectly constant as well! Earth’s not a perfect circle, and it’s tilted, which affects how fast it spins. This means a “day” based on the Sun (solar day) is slightly different from a day based on stars (sidereal day). Leap years help account for this in our calendars.
Around the Sun
Now, as we can understand how to measure the movement and how Earth moves on its axis, we can also understand how Earth moves around the sun and how a relative point makes a difference in this measure.
Well, Earth moves around the Sun in an elliptical path, which means its path is slightly oval-shaped. This takes the Earth 365.25 days to complete one revolution around the Sun, which is why we have a year. This is called Earth’s revolution. Earth revolves around the Sun at a rate of about 67,000 miles per hour (107,000 km/hr or nearly 30 km/s). But now if we take into account the Sun’s movement, then it gets more interesting.
You might think if we are measuring everything in our day-to-day life by taking the sun as our relative point, then it might be constant but in reality, it’s not. The sun is also moving around the Milky Way galaxy, which is also moving throughout the universe, but that might be going too far just to understand how Earth moves so we will stick to the movement of the sun.
So if you’re thinking if the sun moving through the Milky Way galaxy affects us and the movement of Earth or not then, yes, the Sun does indeed orbit around the center of the Milky Way galaxy, and this does have implications for the movement of Earth and the entire solar system. This movement is an elliptical orbit, meaning it’s slightly oval-shaped, not a perfect circle. This journey takes roughly 220-250 million years to complete, making one galactic year also known as a cosmic year. The Sun travels at a speed of 828,000 kilometers per hour. The sun doesn’t move around the Milky Way galaxy as you might think, as that is also different from different relative points. The Milky Way is a huge collection of stars, dust, and gas. It’s called a spiral galaxy because if you could view it from the top or bottom, it would look like a spinning pinwheel. The Sun is located on one of the spiral arms, about 25,000 light-years away from the center of the galaxy.
Impact on Earth’s Movement:
While the Sun does move within the Milky Way, it doesn’t directly affect Earth’s daily rotation or its yearly revolution around the Sun. These motions are governed by Earth’s inertia and the gravitational pull of the Sun, which remain relatively constant even within the larger context of the galactic orbit.
However, there are some subtle impacts on Earth’s movement due to the Sun’s galactic journey:
- Slow speed variation: As the Sun navigates its path, the overall speed of the solar system through space fluctuates slightly. This can influence the length of a year by microseconds, but it’s too little to notice in everyday life.
- Exposure to different regions of the galaxy: Different areas of the Milky Way might have varying densities of interstellar matter (dust and gas). Passing through these regions could potentially influence the frequency of comets and meteoroids entering our solar system, although this effect is still under investigation.
Why Does It Matter?
We all love time-lapse videos of the stars moving across the sky, but really, we are the ones who are tumbling through the universe on a giant wet rock vehicle called Earth, with a windshield called the sky. As viewed from above the North Pole, we spin counterclockwise; west chases east. But we don’t just spin; we also revolve around the Sun on a plane tilted 23.4 degrees relative to our spin. It’s kind of nauseating at this scale, but from this perspective, we can see that the Sun rising and setting is just the Earth pointing you towards and then away from the Sun. This motion causes your sunrise, your noon (the moment when the Sun is highest in your sky), before your sunset. To more closely investigate this movement, let’s talk about meridians. You are on one at this very moment. Your meridian is just aligned from where you are right now straight towards the North and South Poles; it’s a line of longitude, as opposed to the horizontal lines that lay flat when north or south is up, that we call latitude or, actually, flat-attitude.
The Sun is highest in the sky to you, your noon, when your Meridian is pointed right at the Sun. A cool thing happens at this moment: all shadows around you point directly towards one of Earth’s poles, unless you’re on the subsolar point. The subsolar point is the point on Earth’s surface directly below the Sun. It’s always somewhere; you can check its current location online (links as always in the description).
On the subsolar point, shadows fall straight down so they can easily disappear. Twice a year, the subsolar point crosses over Hawaii, the only place in the US where it hits land. And when it does, it is called Lahaina noon, meaning cool Sun.
Straight vertical objects look unnatural during this brief time, like they don’t belong, as if they were photoshopped in without regard for reality. In Honolulu, a sculpture by Isamu Noguchi called “Sky Gate” casts a twisted shadow all day, every day, except during Lahaina noon, when its shadow is a perfect circle.
You may not live in a place where the Sun ever appears directly overhead, but once every Earth rotation, the subsolar point falls somewhere on your Meridian, making it noon for you. The technical name for this noon for you is Local Apparent Solar Noon. The clock on your wrist and the clock on your phone don’t tell you your local apparent solar time because, long ago, we realized that if every meridian had its own time, a person just a few kilometers away, seeing different shadows than you did, would disagree with you on what time it was. So towns adopted their own time. Now, later on, this trick was standardized, and time zones as we know them today came about. This is why it matters because the time we now see on our clock doesn’t actually represent solar time or how far Earth moved around the sun or on its axis; it’s now our time. Even if it’s not perfect, it’s nearly perfect and that is enough.
In conclusion, we can say we know how Earth exactly moves yet are still unwrapping many mysteries of how exactly Earth moves.
