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The Expanding Universe: How the Universe Got Bigger As We Measured It

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Since before history began, we have tried to understand our world and our place in it. To the earliest hunter-gatherer tribes, this meant little more than knowing the tribe's territory. But as people began to settle and trade, knowing the wider world became more important, and people became interested in the actual size of it. Aristarchus of Samos (310-230 BC) made the earliest surviving measurements of the distance between objects in space. By carefully measuring the apparent size of the Sun and Moon and carefully observing the terminator of the Moon when half full, he concluded that the Sun was 18-20 times farther away than the Moon. The actual value is 400, but he was on the right track; he just didn't have precise enough measurements.


A diagram from Aristarchus' work, "On Size and Distances," describing how to work out the relative distances.

Meanwhile, Eratosthenes of Cyrene (276-195 BC) was working on the size of the Earth. He came upon a letter stating that at noon in Syene (modern-day Aswan) on the summer solstice, one could look down a well and see all the way to the bottom because the Sun was precisely overhead. Eratosthenes already knew the distance between Alexandria and Syene, so all he had to do was observe the angle of the Sun on the summer solstice there and then do a little math. Assuming a spherical Earth, he computed the circumference to be 252,000 stadia, which works out to 39,690 km -- which is less than a 2% error compared to the real value. A directly measured size now existed for the world. But what of the heavens? The work of Aristarchus wasn't accurate enough. After figuring out how to reliably predict eclipses, Hipparchus (190-120 BC) used them to get a better estimate of the ratio of distance between Moon and Sun. He concluded that the Moon was 60.5 Earth radii away, and the Sun was 2,550 Earth radii away. His lunar distance was pretty accurate -- that works out to 385,445 km to the Moon, which is pretty close to the actual distance, an average of 384,400 km -- but for the Sun it worked out to 16 million km, about 136 million km short of the actual distance.

Above left: A dioptra, a predecessor to both the astrolabe and the theodolite, of a type similar to the one Hipparchus used to make his measurements.

When Ptolemy (AD 90-168) came along, the Universe shrank for a while.

Using the epicycles he assumed must exist within his geocentric universe, he estimated the distance to the Sun to be 1,210 Earth radii, and the distance to the fixed stars to be 20,000 Earth radii away; using modern values for the Earth's average radius, that gives us 7,708,910 km to the Sun and 127,420,000 km to the fixed stars. Both of those are woefully small (Ptolemy's universe would fit within the orbit of Earth), but they get even smaller if we use his smaller estimate for the Earth's circumference -- he estimated the Earth to be about 1/6 the size it actually is. (And therein hangs a tale, for Christopher Columbus would try to use Ptolemy's figure when plotting his journey west to the Orient, rather than the more accurate ones that had been developed in Persia since then.)


Ptolemy's world; at the time, the best map that existed of the known world.

By the end of the 16th Century, the size of the Earth was pretty well defined, but the size of the Universe remained challenging. Johannes Kepler solved the puzzle of orbital motion and calculated the ratio of the distance between Sun and various planets, enabling accurate predictions of transits. In 1639, Jeremiah Horrocks made the first known observation of a transit of Venus. He estimated the distance between Earth and the Sun at 95.6 million km, the most accurate estimate to date (and about 2/3 the actual distance). In 1676, Edmund Halley attempted to measure solar parallax during a transit of Mercury, but was unsatisfied with the only other observation made. He proposed that further observations be made during the next transit of Venus, in 1761. Unfortunately, he did not live that long.



Jeremiah Horrocks, observing the transit of Venus by the telescopic projection method.

In 1761, acting on the recommendations of the late Edmund Halley, scientific expeditions set out to observe the Transit of Venus from as many places as possible. More expeditions set out in 1769 for the second transit of the pair, including a famous journey by Captain James Cook to Tahiti, and in 1771, Jerome Lalande used the data to calculate the Sun's average distance as 153 million km, far larger than previously estimated, and the first time the measurement was close to right. Further transits in 1874 and 1882 refined the distance to 149.59 million km. In the 20th Century, it has been refined further using radio telemetry and radar observations of the inner planets, but it has not strayed much from that value. The size of the solar system was now known.

Above left: Sketch depicting the transit circumstances, as reported by James Ferguson, a Scottish self-taught scientist and inventor who participated in the transit observations.

But the universe is bigger than the solar system. In the 1780s, William Herschel mapped the visible stars in an effort to find binary stars. He found quite a few, but he also worked out that the solar system was actually moving through space, and that the Milky Way was disk shaped. The galaxy, which was at that time synonymous with Universe, was eventually estimated to be about 30,000 light years across -- an inconceivably large distance, but still far too small.

Hershel's map of the galaxy could not tell how far away any of the stars were; stars get dimmer as they move away, but you can only use this to calculate their distance if you know how bright they are to begin with, and how can you know that? In 1908, Henrietta Leavitt found the answer: she noticed that Cepheid variable stars had a direct relationship between their luminosity and the period of their variation, allowing astronomers to deduce exactly how bright they are to start with. Harlow Shapley immediately applied this discovery and found three amazing things when he mapped all the visible Cepheids: the Sun is actually nowhere near the center of the galaxy, the center of the galaxy is obscured by vast amounts of dust, and the galaxy is at least ten times larger than anyone had ever suspected -- so vast that it would take light 300,000 years to cross it. (Shapley was overestimating a bit; it's actually more like 100,000 light years or so.)

Above left: Henrietta Leavitt, one of the few women in astronomy and the only one on this list; she got little recognition for her discovery at the time.

In 1924, Edwin Hubble produced the next major revolution. Using the new 100-inch telescope at Mount Wilson Observatory, he located Cepheids in the Andromeda Nebula, a spiral nebula in which no stars had previously been resolved. He calculated these Cepheids were 1.2 million light years away, putting them far beyond Shapley's wildest estimate for the size of the galaxy. Therefore, Andromeda was not a part of our galaxy at all; it was an entirely separate "island universe," and most likely the same was true of other spiral nebulae. This meant the Universe was very likely far larger than anyone could hope to measure. It could even be infinite.

At left: The 100-inch telescope at Mount Wilson Observatory, where Hubble did his work. It was the world's largest telescope until 1948.

And then Hubble found something even more astonishing. In 1929, Hubble compared the spectra of near and far galaxies, based on distances already known by observations of Cepheid variables. The spectra of more distant ones were consistently redder, and for nearly all of them, there was a linear relationship between redshift and distance. Due to the Doppler Effect, this meant they were receding. He wasn't sure what to make of this observation at the time, but in 1930, Georges Lemaître pointed out a possible solution: he suggested that the universe was expanding, carrying galaxies along with it, and that at one time it had all be compacted down impossibly tight. Hubble went with this and calibrated the apparent expansion against the distance to known standard candles, calculating the age of the most distant objects to be 1.8 billion light years.

At left: Georges Lemaître, who happened to also be a Catholic priest. He died in 1966, shortly after learning about the Cosmic Microwave Background radiation, which further reinforced his theory of the Big Bang.

This was much too small, and in 1952, Walter Baade figured out why: there are actually two kinds of Cepheids, and Hubble had been observing the ones that Leavitt had not baselined. After characterizing this new population of Cepheids, he recalculated from Hubble's observations and brought the Universe's minimum age up to 3.6 billion years. In 1958, Allan Sandage improved it more, to an estimated 5.5 billion years.

Astronomers started to ratchet up their observations of ever more distant objects. In 1998, studies of very distant Type 1A supernovae revealed a new surprise: not only is the universe expanding, but the rate of the expansion is increasing. Today, the Universe is usually estimated to be 13.7 billion years old -- or, more accurately, the most distant things we can observe appear to be that far away. The catch, of course, is that we're observing them in the past. They're actually further away now -- assuming, of course, that they even still exist. A lot can happen in 13.75 billion years. And now that we know the universe's expansion is accelerating, they are even farther away by now. The current estimate for the actual size of the observable universe is 93 billion light-years in diameter, a tremendous size that the human brain cannot begin to fathom on its own, vastly overwhelming the tiny universe of the ancient Greeks.


NASA artist's concept of the progenitor of a Type 1a supernova -- a neutron star stealing matter from a supergiant companion until eventually enough matter is collected to trigger a supernova.

The understanding of the size of the Universe has gone from being impressed by the distance to the Sun, to the size of the solar system, to the vastness of the galaxy, to the staggering distance to neighboring galaxies, to the mindbendingly complicated distances to things that we can only see as they were an impossibly long period of time ago. What will we discover as we measure the Universe tomorrow?

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Space
Thanks to NASA, the Search for Habitable Worlds Just Got Easier
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NASA

New NASA research will make it easier to find the planets out there that can support life. Detailed in the The Astrophysical Journal, the new model can simulate atmospheric conditions in a more comprehensive way, taking circulation of the atmosphere and other factors into account.

The search for habitable planets requires detailed modeling. The scope of the universe is simply too vast for scientists to spend time searching planet-by-planet. Instead, they calculate factors that would allow a planet to support liquid water—a requirement to support life as we know it—using simulations. For instance, it has to be just far enough from its parent star that the atmosphere isn't so cold that bodies of water freeze, but not so hot that they evaporate.

When planets are losing their oceans due to evaporation, they enter what's called a "moist greenhouse" state as the water vapor rises into the stratosphere and the hydrogen atoms break apart from the oxygen atoms to escape into space, eventually resulting in the loss of the planet's oceans. The new research details how a star's radiation influences how the atmosphere of an exoplanet circulates and plays a role in creating that moist greenhouse state. 

Planets that orbit a low-mass star—the most common kind of star in our galaxy—would have to be closer to that star than the Earth is to the Sun in order to support life, since a low-mass star is cooler and dimmer. The gravity from such a close star would slow down the rotation of the planet, and it might even become locked, with one side perpetually facing the star and one side perpetually facing away. (It would be as if the Eastern Hemisphere were always light and the Western Hemisphere were always dark.)

In turn, the planet would form a thick layer of clouds on the perpetually sunny side. The near-infrared radiation from the star—and cooler stars emit more of this radiation than hotter ones do—interacts with the water vapor in the air and the droplets and ice crystals in the clouds to warm up the air, creating the moist greenhouse state.

The moist greenhouse state could happen even at temperatures as low as those found in the tropical regions of Earth because of that near-infrared radiation interaction, according to the new model, but the study found that in exoplanets close to their stars, the process happens gradually enough that they could remain habitable. This more nuanced model will help guide scientists in their search for habitable planets near low-mass stars.

"As long as we know the temperature of the star, we can estimate whether planets close to their stars have the potential to be in the moist greenhouse state," study co-author Anthony Del Genio explained in a NASA press release. "Current technology will be pushed to the limit to detect small amounts of water vapor in an exoplanet's atmosphere. If there is enough water to be detected, it probably means that planet is in the moist greenhouse state."

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Space
Look Up! The Orionid Meteor Shower Peaks This Weekend
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Ethan Miller/Getty Images

October is always a great month for skywatching. If you missed the Draconids, the first meteor shower of the month, don't despair: the Orionids peak this weekend. It should be an especially stunning show this year, as the Moon will offer virtually no interference. If you've ever wanted to get into skywatching, this is your chance.

The Orionids is the second of two meteor showers caused by the debris field left by the comet Halley. (The other is the Eta Aquarids, which appear in May.) The showers are named for the constellation Orion, from which they seem to originate.

All the stars are lining up (so to speak) for this show. First, it's on the weekend, which means you can stay up late without feeling the burn at work the next day. Tonight, October 20, you'll be able to spot many meteors, and the shower peaks just after midnight tomorrow, October 21, leading into Sunday morning. Make a late-night picnic of the occasion, because it takes about an hour for your eyes to adjust to the darkness. Bring a blanket and a bottle of wine, lay out and take in the open skies, and let nature do the rest.

Second, the Moon, which was new only yesterday, is but a sliver in the evening sky, lacking the wattage to wash out the sky or conceal the faintest of meteors. If your skies are clear and light pollution low, this year you should be able to catch about 20 meteors an hour, which isn't a bad way to spend a date night.

If clouds interfere with your Orionids experience, don't fret. There will be two more meteor showers in November and the greatest of them all in December: the Geminids.

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