Home SCIENCE How recently have we understood the Universe? | by Ethan Siegel | Starts With A Bang! | Nov, 2023

How recently have we understood the Universe? | by Ethan Siegel | Starts With A Bang! | Nov, 2023


This tiny region of the JADES survey shows a mix of galaxies: some that are relatively nearby, large, highly evolved, and massive; others that are at intermediate distances and have a mix of old-and-young stars in them, and a great number of very distant or even ultra-distant galaxies that are faint, heavily reddened, and potentially from the first 5% of our cosmic history. In this one little region, the power of JWST, and the evolution of the angular scale of the Universe, is on full display. Views like this, of the Universe, were unfathomable just a few short decades ago. (Credit: NASA, ESA, CSA, STScI)

While humanity has been skywatching since ancient times, much of our cosmic understanding has come about only recently. Very recently.

Since ancient times, humanity has studied the skies.

70,000 years ago, a red dwarf-brown dwarf pair known as Scholz’s star, so faint that it was only discovered very recently, passed through the Solar System’s Oort cloud. Unlike the illustration, however, it’s so intrinsically faint that it still wouldn’t have been visible to human eyes; today, it’s approximately 22 light-years away. Other stars, in the near future, will pass even closer, but it is a near-certainty that even our pre-human ancestors were watching and cataloguing the night skies. (Credit: José A. Peñas/SINC)

Cometary sightings, eclipses, and “temporary” stars date back thousands of years.

This particular image contrasts the constellations of the sky as they appeared thousands of years ago with corresponding artwork carved into stone at around the same time. Evidence that ancient peoples had an advanced knowledge of astronomy can be traced back as far as 38,000 years ago from cave paintings and other archaeological evidence. (Credit: M. Sweatman & A. Coombs, Athens Journal of History, 2018)

Some ~2800 years ago, the Babylonians already predicted eclipses.

When the Moon passes directly between the Earth and the Sun, a solar eclipse occurs. Whether the eclipse is total or annular depends on whether the Moon’s angular diameter appears larger or smaller than the Sun’s as viewed from Earth’s surface. Only when the Moon’s angular diameter appears larger than the Sun’s are total solar eclipses possible, a situation that will no longer be possible about 600–650 million years from now. Eclipses have been predictable phenomena for nearly 3000 years: since the time of the ancient Babylonians. (Credit: Kevin M. Gill/flickr)

By the 3rd Century BCE, Eratosthenes measured a spherical Earth’s circumference.

If the Earth were perfectly flat, then the Sun’s rays would cast identical shadows at noon on the solstice everywhere on Earth (top), no matter where you were located. But if the Earth’s surface were curved (bottom), shadows at different locations would cast different shadows on the same day, depending on the angle that the Sun’s rays struck the object in question. By measuring the difference in shadow angle between two points on Earth’s surface, it became possible to not only determine Earth’s spherical (or spheroidal) nature, but to measure the size of the Earth for the first time. (Credit: E. Siegel/Beyond the Galaxy)

Shortly thereafter, Aristarchus estimated the Moon’s size and distance.

Animation showing the umbral phase of the November 19, 2021 partial lunar eclipse. At 9:03 AM UT, maximum eclipse is reached, where only 0.9% of the Moon remains illuminated by direct sunlight. The umbral phase lasts over 3.5 hours: the longest this century for a partial eclipse. Reconstructing the size of Earth’s shadow relative to the physical size of the Moon is the oldest method for measuring both the size of the Moon as well as the distance to it: a method first leveraged by Aristarchus back in the 3rd Century BCE. (Credit: NASA’s Scientific Visualization Studio)

But making sense of the Universe took much longer.

Artist’s logarithmic scale conception of the observable universe. The Solar System gives way to the Milky Way, which gives way to nearby galaxies which then give way to the large-scale structure and the hot, dense plasma of the Big Bang at the outskirts. Each line-of-sight that we can observe contains all of these epochs, but the quest for the most distant observed object will not be complete until we’ve mapped out the entire Universe. (Credit: Pablo Carlos Budassi; Unmismoobjetivo/Wikimedia Commons)

Planetary motion — in ellipses around the Sun — wasn’t known until Kepler’s 1609 discovery.

Tycho Brahe conducted some of the best observations of Mars prior to the invention of the telescope, and Kepler’s work largely leveraged that data. Here, Brahe’s observations of Mars’s orbit, particularly during retrograde episodes, provided an exquisite confirmation of Kepler’s elliptical orbit theory. Kepler put forth his 1st and 2nd laws of planetary motion in 1609, with his 3rd law coming 10 years later: in 1619. (Credit: Wayne Pafko)

The recurrence of comets wasn’t understood until Edmond Halley: in 1705.

This comet, imaged in 2015 and known as C/2014 Q2 Lovejoy, brightened sufficiently to become as bright as magnitude +4: visible to the naked human eye even under fairly light-polluted conditions. Comet Swift-Tuttle, when it next returns, will be about 20 times brighter, and is far more massive and dangerous. Although comets have been recorded for thousands of years, their periodic nature was only uncovered in the 18th century, by Edmond Halley. (Credit: John Vermette / MIT News)

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