The commonly used Earth-based distance unit of "kilometers" is insufficient in outer space. Within our solar system, the distance from Earth to the Sun, approximately 150 million kilometers, is a distance measurement unit known as an "astronomical unit."

For instance, the distance from Mars' orbit around the Sun ranges from about 1.38 to 1.66 astronomical units.

However, when we venture beyond the solar system into the vastness of outer space, a larger unit of measurement called a "light-year" is utilized. With light traveling 300,000 kilometers per second, roughly the distance from Earth to the Moon can be covered in about a second. Consequently, the distance light travels in one year is an unimaginably vast measure.

For example, the closest star to the Sun, Proxima Centauri, is approximately 4.2 light-years away. This means that the light we see from Proxima Centauri today began its journey 4.2 years ago.

Now, why is it possible to see galaxies millions or even billions of light-years away but not discern planets within one light-year? This question involves two distinct concepts: "seeing" and "seeing clearly." Observing planets within one light-year presents greater challenges compared to seeing galaxies. Human technology and the time involved are crucial factors enabling our ability to follow galaxies billions of light-years away.

Humanity has developed various technologies to observe distant galaxies, ranging from ground-based optical and radio telescopes to space telescopes launched into orbit. These technologies enable capturing extremely faint light, allowing us to obtain images of distant galaxies. Astronomers analyze these images to calculate the distances to these galaxies.

Using extended exposure times techniques, instruments like the Hubble Space Telescope have glimpsed incredibly distant cosmic pasts. Galaxies billions of light-years away essentially represent their state billions of years ago.

It takes time to travel from the plant to our taps, like water reaching our homes from a water plant. Similarly, light from distant objects takes time to contact us. In the future, if humanity invents faster-than-light spacecraft, traveling a thousand light-years and returning to Earth would mean stepping into a different era—experts suggest this understanding about time and space in the universe.

According to Einstein's theory, when light reaches the speed of light, time freezes. Therefore, one might expect the light we see to be produced instantly. Everything we see is due to the light that reaches our eyes. Often, what we perceive as "seeing" is actually discerning the light actively entering our eyes.

Light has speed, but because it's extremely fast—approximately 300,000 kilometers per second—we don't perceive the differences over short distances, making it seem synchronous with our observations. However, it's not the case. Similar to thunder and lightning, where the sound of thunder reaches us after the lightning, light behaves similarly.

When you see an object one kilometer away, you see it as 1/300,000 seconds ago. If you gaze at the Moon, approximately 380,000 kilometers away, you see it as it was 380,000/300,000 seconds ago. The notion of "seeing is believing" is flawed because what you see is not in the present universe—it's a tiny fraction of time in the past.

The farther the distance, the more apparent this phenomenon becomes. For instance, when you witness the sun rise above the horizon, you see it as it was eight minutes ago; its actual position should be where it will be eight minutes from now. Many of the stars we observe today have already ceased to exist—what we see is their 'essence,' not their present form.