- February 28, 2024 - Planétarium : Experience
The Earth is our natural spaceship, and carries us along on its journey through space. Its movements provide the rhythm for our measurement of time, however much our daily lives seem totally different from celestial mechanics. And yet, the rotation of our planet dictates how long a day lasts, and its revolution around the Sun determines the length of a year.
On a daily basis, it’s conceivable to reconnect with the Earth’s rotation and the length of the day. Hour after hour, our planet’s rotation causes the Sun’s position to shift over our heads, replacing it at night with stars to produce the cycle of day and night. In a sense, our day-to-day lives are closely linked to the Earth’s rotation, whether we’re aware of it or not.
Observing the revolution of the Earth around the Sun, one of the causes of the change in seasons, requires patience and preparation. You can keep an eye on the change in the star’s position relative to the landscape that surrounds us. Here’s a brief guide to help you do that.
Stage 1: Define your observation space
On a cloudless night, choose a good spot for observation. That could be a balcony, a window, a park… It’s not necessary to leave your home. What’s important is that you be able to see part of the sky with an Earth element in the foreground: a tree, a rooftop, or a building. For the best experience, make sure not to be facing north.
Stage 2: Locate “your” star
Pinpoint a bright star not far from your foreground reference point. You don’t even have to know the name of that star! Carefully note its position with regard to a visible earthly element (e.g., my star is at such a distance from the wall of this building). Careful: the experiment doesn’t work with the Moon or a planet. If in doubt, consult the map of the Monthly Sky to distinguish planets from stars.
Stage 3: Pay close attention to your star
Every evening when weather permits, place yourself exactly at the same spot at the same time and note the position of your star relative to your reference point. From one evening to the next the difference will be minimal, but over the course of time you’ll notice that the star doesn’t maintain the same position as on previous evenings.
It may not seem spectacular, but you’ve nevertheless just indirectly observed the slight daily displacement of the Earth along its orbit compared to the day before.
Explanations
For this exercise, we can consider that the stars are so far away from us that, even though they’re in motion, it will take hundreds – even thousands – of years before we can notice a significant change in their position with respect to one another. If a star seems to move with regard to a reference element, that’s because the entire celestial sphere undergoes the same movement.
By observing the sky very exactly at the same time one evening after the other, we’re choosing to look along a sightline that is always oriented the same way in relation to the Sun, the reference point for the 24-hour rotation. Each evening we verify whether the sightline is pointing in the same direction of the sky. Over the course of the experiment, we’ll determine that our star is moving little by little in relation to our foreground earthly element (building, tree, whatever).
When we always observe at the same time of day, to guarantee the same configuration of the Earth-Sun pair, the only plausible explanation is that the entire celestial sphere has moved in relation to the Earth-Sun pair. That movement, circular and cyclical (which repeats roughly every 365 nights), corresponds to the revolution of the Earth around the Sun – although from our point of view, it’s the celestial sphere that appears to revolve around us.
