When you look at a distant galaxy through a telescope, how is it that you're looking backward in time?
This is because the speed of light is finite: c = 299792458 m/s
Ole Christensen Rømer in about 1670 actually found out that the speed of light was finite (and estimated its value), by observing the moons of jupiter. The distance between Earth and jupitar changes, as both the Earth and Jupiter revolve around the Sun but with different rotational periods.
The moons of jupiter have some fixed orbital period, so you can predict where they will be relative to jupiter. However, because the speed of light is finite, you will observe them with some delay that depends on the distance between the Earth and Jupitar. This delay changes over the course of a year, and from this you can compute the speed of light.
Rømer could only estimate the ratio of the lightspeed and the Earth's speed in its orbit around the Sun, because at that time the overall scale of the solar system was unknown (only the ratio of distances to Earth-Sun distance was known then)
When you observe a distant galaxy through a telescope, you are indeed looking backward in time. This is due to the finite speed of light.
Light travels at a constant speed of approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum. When the light from a distant galaxy reaches your telescope, it has traveled across vast distances to reach your eyes.
Since it takes time for the light to travel from the galaxy to Earth, the light you are seeing left the galaxy at a particular point in the past. The farther away the galaxy is, the longer it takes for its light to reach us. This means that the light you observe has traveled for thousands, millions, or even billions of years before reaching your eyes.
Therefore, when you observe a distant galaxy, you are essentially observing it as it looked in the past, allowing astronomers to study objects and phenomena that occurred at different points in cosmic history. This phenomenon is often referred to as "lookback time" or "cosmic time."
When we observe a distant galaxy through a telescope, we are indeed looking backward in time, and this is because light takes time to travel from objects in space to our eyes or our telescopes.
Light is the fastest thing in the universe, but it still takes time to travel through space. The speed of light is approximately 299,792 kilometers per second. Since the universe is enormous, it can take a considerable amount of time for light to reach us from distant objects.
Let's consider a scenario: Imagine you are observing a galaxy located 10 million light-years away. This means that the light you're seeing from that galaxy today actually started its journey 10 million years ago. So, when you observe the galaxy, you are seeing it as it appeared 10 million years ago.
This phenomenon is known as the "light travel time." The farther the object is from us, the longer the light takes to reach our telescopes. Therefore, observing distant objects in space allows us to peer back into the past and study how the universe looked at earlier stages of its history.
In summary, when we look at a distant galaxy through a telescope, we are seeing it as it was in the past because the light from that galaxy took time to reach us.