Introduction

Every astronomical observation is also an observation of the past.
Because light travels at a finite speed, the images we receive from celestial objects show them not as they are now, but as they were when their light began its journey toward Earth.

This article explores what that means in practice — from nearby objects within our solar system to distant galaxies — and asks a simple but profound question: how far back in time can we see through a telescope?

It is important to note that this is not time travel, but a direct consequence of the physical properties of light and space.

Earth–Moon Distance

Let us begin close to home.

The average distance between the Earth and the Moon is approximately 400,000 km, also referred to as one lunar distance (LD). Light takes about 1.28 seconds to travel this distance.

To put this into perspective:

  • Earth’s diameter is about 12,742 km
  • The Earth–Moon distance is roughly 30 Earth diameters
  • The actual distance varies from about 356,500 km at perigee to 406,700 km at apogee

Even when observing the Moon, we are therefore seeing it just over one second in the past.

Distance of One Light Year

A light year is the distance light travels in one year, approximately:

  • 9.46 trillion kilometers (≈ 10¹³ km)

In astronomy, distances are often expressed in parsecs (pc):

  • 1 parsec ≈ 3.26 light years
  • 1 parsec ≈ 206,265 astronomical units (AU)

As an example, the Pleiades (Messier 45) are located about 400 light years away.
When we observe them today, we see them as they appeared around the year 1625.

Astronomical Units

Within our solar system, distances are often expressed in astronomical units (AU).

  • 1 AU ≈ 150 million km
  • Light travels this distance in about 8 minutes

For comparison:

  • Moon: ~400,000 km
  • Sun: ~150 million km
    → The Sun is about 375 times farther away than the Moon

Distances to Selected Deep-Sky Objects

To illustrate cosmic time scales, here are approximate distances to well-known objects:

  • Pleiades (M45): 400 light years
  • Beehive Cluster (M44): 577 light years
  • X Persei: ~7,500 light years
  • Double Cluster (h and χ Persei): ~7,200 light years
  • Hercules Globular Cluster (M13): ~22,000 light years
  • Andromeda Galaxy (M31): ~2.5 million light years
  • Messier 66: ~31 million light years
  • Messier 87 (Virgo A): ~53 million light years

When we observe these objects, we are directly witnessing earlier epochs of cosmic history.

Observation, Resolution, and Distance

Seeing farther does not necessarily mean seeing more detail.

What can be resolved depends on several factors:

  • distance
  • intrinsic brightness
  • angular size
  • wavelength
  • atmospheric conditions
  • telescope aperture and optics

A nearby faint object may be harder to observe than a distant but intrinsically luminous one.

This article explores what that means in practice — from nearby objects within our solar system to distant galaxies — and asks a simple but profound question: how far back in time can we see through a telescope?

It is important to note that this is not time travel, but a direct consequence of the physical properties of light and space.

Age of the Universe and the Role of Space Telescopes

The age of the universe is estimated at approximately 13.8 billion years.

With a high-quality amateur refractor, it is possible to image galaxies such as Messier 66, located about 31 million light years away, while still clearly identifying their galactic structure.

Space-based observatories such as the James Webb Space Telescope extend this reach dramatically.
JWST is designed to observe the infrared light of the universe’s first stars and galaxies, formed more than 13.5 billion years ago.

In practical terms:

  • Amateur telescopes allow us to see tens of millions of years into the past
  • Modern space telescopes allow us to observe the early universe itself

Conclusion

Looking through a telescope is not merely an act of observation — it is an encounter with time.

From the Moon’s reflected light to distant galaxies whose photons began their journey long before human history, astronomy offers a unique and direct connection to the past.

The farther we look into space, the deeper we look into cosmic history — a perspective that remains both scientifically profound and deeply humbling.

Sources and References