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

The world tells us its story greatly through light and other wavelength of electromagnetic radiation. Us learn around the planets, stars, and galaxies by your light—visible light, and also shorter-wavelength ultraviolet and longer-wavelength infrared light, invisible come the eye but detectable by certain telescopes ~ above Earth and in space—and by the still longer waves the radio energy that they send us. These waves carry out not come instantaneously. Although they take trip at the fastest feasible speed (the speed of light,) they take it a when to gain here. The world is big, for this reason the news is delayed by the substantial gulfs of room it has to cross to with us. Irradiate covers 186,000 miles EVERY second (kids, you re welcome don"t shot traveling this fast without adult supervision!!!) In metric units, that"s about 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 exactly how long its light takes to reach us here on Earth.

The Moon and the Sun


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

The closest object to us is the Moon. Its typical distance is about 240,000 miles, so irradiate from the Moon take away (240,000 miles divided by 186.000) 1 and 1/3 seconds to gain from the Moon come Earth. Once astronauts orbited the Moon and also later to walk on its surface in the 1960"s, television viewers noticed the they were slow to answer questions transmitted native Earth. The was due to the fact that it take it 1.3 seconds for the question to travel to the Moon, and another 1.3 secs for the answer to get ago to Earth. Those 2.6 seconds were precisely the round-trip travel time because that radio waves in between the Earth and also the Moon.

The Sun is 93 million mile away, so sunlight takes 8 and 1/3 minutes to obtain to us. Not lot changes about the sunlight in so quick a time, however it still means that once you look in ~ the Sun, you watch it as it was 8 minute ago. PhotoPhoto that the sunlight in hydrogen-alpha light.

The Planets


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

The large planet Jupiter, whose large moons Galileo uncovered with his "trouble-making" telescope, is much more than 5 times farther indigenous the sun than the earth is. We check out a planet like Jupiter due to the fact that its light—which choose the various other planets and the Moon originates ~ above the Sun—takes around 43 minutes to with Jupiter. The return pilgrimage to the planet can take it from 35 come 52 minutes, relying on whether we are on the same side that the sunlight as Jupiter or ~ above the other side.

Little Pluto is so small and remote it to be not found until 1930, orbits 40 time farther from the sun than we do. Irradiate from the sun takes around 5 and 1/2 hours to reach it and roughly the same time to return to Earth. By the time the light reaches us, it has spread the end so much that the earth looks very dim, and requires a an excellent telescope to spot. PhotoPhoto the Pluto and also its moon Charon, together seen with the Hubble space Telescope in 1994.

Beyond the Solar System

Moving past the Solar System, our range of distances and also travel times needs to change. Currently light will call for years, no hours, to do its means to us. The star the is nearest to the sun happens come be part of a mechanism of 3 stars. (Unlike the Sun, i m sorry is a loner, plenty of stars are uncovered in teams of two, three, 4 or more.) The brightest star in our neighbor system is dubbed Alpha Centauri (pronounced Al" fa Sen" come ree), and also it is a digital twin of the Sun. Irradiate from Alpha Centauri takes much more than 4 years to reach the Sun. (Astronomers use a distinct term because that this method of measure distance—they speak the star is 4 light years away.)


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The dual star Albireo, photographed v a telescope through Timothy Ferris (from seeing in the Dark), is 385 light years native Earth, so the light we see from it tonight was emitted in the seventeenth century.

The brightest star in our skies is the "dog star", Sirius (pronounced Sea" ree us). It"s the main star in the constellation the the large dog, Canis Major. Sirius is roughly 9 irradiate years away. Think that what you were doing 9 years ago. That"s when the irradiate we view from Sirius tonight first began its trip to us. Not much from Sirius in the sky is the shining star Betelgeuse (pronounced Beetle" juice). That is so much that its irradiate takes 430 year to reach us. Light that we see tonight indigenous Betelgeuse left that in the late 1500"s.

In the same component constellation, Orion, as Betelgeuse however even farther away is the Orion Nebula, a location where us see brand-new stars forming. Its street is 1500 irradiate years, an interpretation that the light we see from the left an ext than a thousands years before the invention of the telescope.


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The aside from that away an item in an are lies, the much longer it takes its irradiate to get to us and also the older that light is as soon as it get Earth. Together we look deeper and deeper right into the Milky means Galaxy (the island of stars in which us live), we room looking deeper right into the past. Light have the right to take 10s of hundreds of years or much more to reach united state from far-off parts of our galaxy, i m sorry is approximately 100,000 irradiate years wide.

Other Galaxies

Once we move outside the galaxy, us encounter also larger spaces and also longer light travel times. Among the an excellent scientific concepts of the 20th century astronomy was the discovery that over there are other galaxies out there—stretching as much as our an excellent telescopes have the right to see. Billions of various other islands the stars room scattered with the an excellent dark s of space.


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The nearest large galaxy come the Milky way is the Andromeda Galaxy. Periodically astronomers speak to it M31, through its number in the famous Messier directory 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 this evening left it an ext than 2 million years ago, once our varieties was just start to develop its delicate foothold on planet Earth.

In this sense, astronomy is mostly old history: The farther away objects are, the enlarge the story they need to tell us. Young people, elevated on CNN, the Web, and also "instant messaging" might at an initial bridle at the thought that the most recent info we can obtain from a ar galaxy can be 2 million years old. Yet for astronomers, this delay in the arrival of light is among the universe"s best gifts.

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After all, one of the fundamental tasks the astronomy is to to fill in the history of the universe—from the big Bang come the moment you are analysis this paragraph. Astronomers might not have the ability to undertake such a job if the details from the universe were limited to current events. But the universe is a time machine. Feather at more distant objects, we discover about an ext ancient times and phenomena. Huge telescopes permit us come look billions of years right into the past and to reconstruct the story the the cosmos eon through eon.

Lookback Times because that Some astronomical Objects thing Time because that the irradiate to with 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 our sky) 9 years
Betelgeuse (bright star) 430 years
Orion Nebula 1500 years
Andromeda Galaxy 2.5 million years

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