"Everything we see in the skies belongs to the past."Timothy Ferris, seeing in the Dark (p. 116)

The world tells us its story mostly through light and other wavelength of electromagnetic radiation. Us learn around the planets, stars, and galaxies by their light—visible light, and also shorter-wavelength ultraviolet and longer-wavelength infrared light, invisible to the eye however detectable by specific telescopes top top Earth and also in space—and by the still much longer waves that radio energy that lock send us. These waves execute not arrive instantaneously. Return they travel at the fastest possible speed (the speed of light,) they take a when to acquire here. The cosmos is big, therefore the news is delayed by the vast gulfs of an are it has to cross to with us. Light covers 186,000 mile EVERY second (kids, please don"t try traveling this rapid without adult supervision!!!) In metric units, that"s around 300,000 kilometers per second.

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How long does light require to reach united state from acquainted objects? Let"s take a rapid tour the the solar system, asking at every place just how long its irradiate takes to reach us here on Earth.

The Moon and the Sun


The Moon, photographed with a digital camera through a telescope through Timothy Ferris (from seeing in the Dark).

The closest thing to us is the Moon. Its typical distance is around 240,000 miles, so irradiate from the Moon takes (240,000 miles divided by 186.000) 1 and also 1/3 seconds to gain from the Moon come Earth. Once astronauts orbited the Moon and later walked on its surface in the 1960"s, television viewers noticed that they were sluggish to answer inquiries transmitted indigenous Earth. The was because it took 1.3 secs for the question to travel to the Moon, and another 1.3 seconds for the answer come get earlier to Earth. Those 2.6 secs were exactly the round-trip travel time for radio waves between the Earth and the Moon.

The Sun is 93 million miles away, so sunlight takes 8 and 1/3 minutes to get to us. Not lot changes around the sun in so short a time, but it still means that when you look at the Sun, you view it together it was 8 minutes ago. PhotoPhoto that the sun in hydrogen-alpha light.

The Planets


Jupiter, photographed through an HDTV camera with a backyard telescope by Kenneth Crawford and Michael A. Mayda (from see in the Dark).

The huge planet Jupiter, whose big moons Galileo discovered with his "trouble-making" telescope, is more than 5 times farther indigenous the sun than the earth is. We see a earth like Jupiter because its light—which choose the other planets and the Moon originates top top the Sun—takes around 43 minute to reach Jupiter. The return pilgrimage to the earth can take from 35 to 52 minutes, relying on whether we are on the very same side that the sunlight as Jupiter or on the various other side.

Little Pluto is so tiny and far it to be not found until 1930, orbits 40 times farther indigenous the sunlight than we do. Irradiate from the sunlight takes around 5 and 1/2 hours to with it and also roughly the same time to return to Earth. By the time the irradiate reaches us, it has actually spread out so much that the planet looks an extremely dim, and also requires a good telescope to spot. PhotoPhoto that Pluto and also its moon Charon, together seen through the Hubble an are Telescope in 1994.

Beyond the Solar System

Moving beyond the Solar System, our scale of distances and also travel times requirements to change. Now light will call for years, no hours, to make its method to us. The star that is nearest come the sun happens come be component of a system of 3 stars. (Unlike the Sun, i beg your pardon is a loner, plenty of stars are found in teams of two, three, 4 or more.) The brightest star in ours neighbor device is referred to as Alpha Centauri (pronounced Al" fa Sen" come ree), and also it is a online twin the the Sun. Light from Alpha Centauri takes more than 4 years to reach the Sun. (Astronomers usage a special term for this method of measure up distance—they to speak the star is 4 irradiate years away.)


The twin star Albireo, photographed v a telescope through Timothy Ferris (from see in the Dark), is 385 light years native Earth, for this reason the irradiate we watch from it tonight was emitted in the ten century.

The brightest star in our skies is the "dog star", Sirius (pronounced Sea" ree us). It"s the major star in the constellation of the huge dog, Canis Major. Sirius is approximately 9 light years away. Think of what you were doing 9 years ago. That"s as soon as the irradiate we watch from Sirius tonight first began its journey to us. Not much from Sirius in the sky is the glowing star Betelgeuse (pronounced Beetle" juice). The is so much that its irradiate takes 430 year to reach us. Light the we watch tonight indigenous Betelgeuse left it in the so late 1500"s.

In the same part constellation, Orion, as Betelgeuse yet even farther far is the Orion Nebula, a place where we see brand-new stars forming. Its distance is 1500 light years, definition that the irradiate we check out from the left more than a thousands years before the innovation of the telescope.


The furthermore away things in an are lies, the longer it bring away its irradiate to gain to us and also the older that light is when it get Earth. As we look deeper and deeper into the Milky way Galaxy (the island of stars in which we live), we are looking deeper right into the past. Light have the right to take tens of hundreds of years or an ext to reach us from distant parts of ours galaxy, i m sorry is about 100,000 irradiate years wide.

Other Galaxies

Once us move exterior the galaxy, us encounter even larger spaces and also longer light travel times. One of the great scientific concepts of the 20th century astronomy to be the exploration that over there are various other galaxies out there—stretching as far as our an excellent telescopes have the right to see. Billions of other islands the stars are scattered with the good dark ocean of space.


The nearest large galaxy to the Milky method is the Andromeda Galaxy. Occasionally astronomers contact it M31, by its number in the well known Messier brochure of fuzzy celestial objects. The Andromeda (pronounced an drah" mid a) Galaxy lies about 2 1/2 million irradiate years indigenous Earth. The light we view from it tonight left it an ext than 2 million years ago, as soon as our types was just beginning to create its fragile foothold on planet Earth.

In this sense, astronomy is mostly old history: The farther far objects are, the older the story they have to tell us. Young people, raised on CNN, the Web, and also "instant messaging" might at first bridle at the believed that the most recent information we can gain from a neighbor galaxy can be 2 million year old. However for astronomers, this delay in the arrival of light is among the universe"s best gifts.

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After all, among the an essential tasks of astronomy is to fill in the background of the universe—from the huge Bang to the minute you are reading this paragraph. Astronomers can not be able to undertake such a task if the details from the cosmos were limited to existing events. However the universe is a time machine. Feather at much more distant objects, we find out about an ext ancient times and phenomena. Large telescopes enable us come look billions the years right into the past and also to reconstruct the story the the cosmos eon by eon.

Lookback Times because that Some huge Objects thing Time because that the light to reach Us
The Moon 1 1/3 sec
The Sun 8 minutes
Jupiter 35 to 52 minutes
Pluto 5 1/2 hrs (on average)
Alpha Centauri (nearest star system) 4.3 years
Sirius (brightest star in ours sky) 9 years
Betelgeuse (bright star) 430 years
Orion Nebula 1500 years
Andromeda Galaxy 2.5 million years

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