Hot answers tagged

55

The sun and the moon go around the observer once a day, Eratosthenes knew that the apparent size of moon doesn't change. This must mean that Alexandria is near the centre of the moon's orbit. But the apparent size also doesn't change when viewed from anywhere. So everywhere is close to the centre of the moon's orbit. Thus the moon must be much further ...


33

I think the definitive work is that of Hoyle & Fowler (1960). They argued that supernovae were produced by two possible mechanisms - what they called an implosion/explosion or an explosion within degenerate matter. Both of these mechanisms required very high internal temperatures ($>2\times 10^{9}$ K) and they argued that this could only be achieved ...


31

How was the mass of Venus measured for the first time? In the mid 19th century, Urbain Le Verrier's predicted of the existence of a then unknown planet beyond the orbit of Uranus. He even predicted this planet's orbit. The discovery of Neptune based on his predictions was perhaps his greatest accomplishment. Le Verrier then went on to investigate Mercury. ...


29

Exactly how Eratosthenes calculated the radius of the Earth has been lost. What is presently taught as his method is a simplified version described by Cleomedes. It is unlikely that Eratosthenes assumed the Sun was infinitely distant, since he apparently also estimated the distance to the Sun himself. In any case, his work came after that of Aristarchus who ...


28

Ceres, Pallas, Juno and Vesta were all discovered between 1801 and 1807. After that, astronomers looked in vain for 38 years until the 5th, Astraea was spotted on December 8, 1845 by German amateur astronomer Karl L. Hencke by accident. He stumbled on Astraea while looking for Vesta one night. So, what took so long to discover Astrea? There is a paper1 which ...


20

Here's part of the sky in the year 1 It is part of the sky you may know well, Orion and the dogs. I've marked the current positions of Sirius, Procyon and Betelgeuse, with green markers so you can see how their positions have changed over 2000 years. It's not a lot. The first thing that the Babylonian astronomer might notice is that there is a pole star, ...


16

The mass of Venus was determined by weighing the Earth, or more precisely, by determining the ratio of the density of the Earth to the density of Schiehallion, and assuming Schiehallion to be typical rock of 2500 kg per cubic meter. Prior to that, Jérôme Lalande had worked out the relative masses of the major bodies of the Solar System as a byproduct of ...


16

Although various astronomers have speculated that the Sun was a star (some were imprisoned or even burnt alive for such heresy), this was not known definitively until 1838 when Friedrich Bessel used parallax to calculated the distance to 61 Cygni. In the late 19th century, Lord Kelvin provided rather small (less than 100 million years) for the ages of the ...


15

It looks like the Mariner 9 era. Based on a review paper by Watters et al. ("Hemispheres Apart: The Crustal Dichotomy on Mars"): "The north-south asymmetry ... was clear from the first global image mosaics of Mars returned in the 1970s by Mariner 9 (Mutch et al., 1976) and the Viking Orbiters." Other papers cite a 1973 paper by Hartmann ...


11

I'm pretty sure Newton was the first to apply the notion of the barycentre to celestial motion, although the concept of a centre of mass may predate him. Before Newton, there was a very strong demarcation between celestial motion and terrestrial motion, inherited from the Greek philosophers like Aristotle. (Similar philosophies applied outside the Western ...


11

Länge und Breite are the German words for longitude and latitude. Thus 'b' seems like a natural choice for latitude and 'l' for longitude. A century ago when traditional choice of variable names were chosen by vote of feet, a reasonable amount of research papers were still published in German or people with some form of fluency in German. At the same time it'...


11

15 years. Hubble was designed with an anticipated 15-year lifetime based on the expected integrity of the main mirror. It was believed that over HST’s 15-year life the space environment in low Earth orbit would cause sufficient degradation of the mirror that the telescope’s light-gathering capabilities would be severely damaged by cosmic rays and orbital ...


9

In "The Planet of Doubt", Stanley G. Weinbaum, Astounding Stories, October 1935, there is a scene where Hamilton Hammond, leader of a expedition to the north pole of Uranus, explains his descision to set a southeastern course while searching for land: "I'll tell you. Did you ever look at a globe of the Earth, Pat? Then maybe you've noticed ...


8

Short Answer: Kepler expressed his laws with the sun at a foci rather than a barycenter. Long Answer: In Astronomia Nova (pub. 1609) Kepler presents the first version of something we can recognize as Kepler’s first law: On page 285 of Astronomia Nova: CAPVT LIX. Demonftratio, quod orbita MARTIS, librati in diametro epicycli, fiat perfecta ellipfis: Et ...


8

You may want to look into the Chandrasekhar limit. Dr. Chandrasekhar did his original work on this back in the early 1930's but received the Nobel Price in Physics in 1983. On Stars, Their Evolution and Their Stability


7

There are a few ways to think about this question: Do the stars change positions in the sky ... such that the layout of say... major stars in constellations appear to move over time? Are there events that cause sudden changes (changes you might notice overnight ... or within a few weeks or months)? How long does it take to notice a change? Precession The ...


7

Earth passed through the tail of Halley's Comet in 1910. It caused a bit of a panic due to claims that cyanogen detected in the tail would cause harm: https://en.wikipedia.org/wiki/Halley%27s_Comet#1910 Linked there is https://timesmachine.nytimes.com/timesmachine/1910/02/08/104920328.pdf (click for larger)


7

This is an interesting question, and I think it can productively be split into a couple of distinct parts, separating the publisher side from the author side: Was/is there any pushback from journals about posting pre-publication versions of papers on the web? Why do research article authors in astronomy post papers on arxiv at a relatively high rate? Part ...


7

Yes, on one of the final orbits it took some pictures of the rings while crossing the ring plane: More details of that image are here, and this page show some still images. Here’s another one: though it’s hard to interpret without reading the description. All the science done during the “Grand finale” orbits is described here.


6

I'm going to be vague on the timing because it depends on what you mean by "determine". Which in turn depends on what is meant by "know" in science. There is a very good model for stars which predicts that balls of plasma that are prevented from collapse by fusion in their cores will look like the sun up close and will look like stars ...


5

Discovery of Titan, 1655: Unknown diameter. Dollfus, 1970: 4,850$\pm$300km (1). Measured by Filar micrometer (2) and diskmeter / double-image micrometer (3). (Apparently a summary of earlier measurements, currently trying to find print copy) NASA SP-340, 1974: Summary of above techniques, propose settling on 5,000km diameter until it can be measured by ...


5

A small portion of Eugene Shoemaker's ashes were placed on the Lunar Prospector mission. After completion of the mission's primary objectives: Lunar Prospector was deliberately impacted onto the shadowed Shoemaker crater on the lunar surface at 09:52:02 UT July 31, 1999. NASA Mission Description No coordinates are given in that source, however, the Google ...


5

The biggest systematic change is caused by the precession of the earth's orbit, with a period of about 26,000 years. To observe and deduce anything about that relies on record keeping. There is no hard evidence that the ancient Babylonian or Egyptian astronomers kept any relevant long term records. The earliest known discovery of this phenomenon is ...


5

It would be my understanding that the work by Hoyle in the late 40s to early 60s established a mechanism by which massive stars could supernova, and so explain the "Type II" supernova that had been observed. It would have been apparent from the start that the sun was no a "massive star". So since the late 40s/early 50s, it has been known ...


5

The quotation is probably referring to The Great Comet of 1861. According to this Wikipedia article: https://en.wikipedia.org/wiki/C/1861_J1 The comet may have interacted with the Earth in an almost unprecedented way. For two days, when the comet was at its closest, the Earth was actually within the comet's tail, and streams of cometary material converging ...


5

Precession of the equinox, caused by the cyclical change in the direction of the Earth's tilt, causes the position of the vernal equinox (the point where the path of the sun crosses the celestial equator) to move over time. When the constellations were first described, the vernal equinox was in Aries, and so it is still called "The First Point of Aries&...


5

It's surprisingly hard to research this, partly because the name "H-alpha" only became common towards the end of the 19th Century, and partly because (I get the impression) a lot of mid- and late-19th Century spectroscopy ignored the red end of the spectrum, perhaps because the equipment was less sensitive there. Thus, you can find a fair amount of ...


4

It depends on many factors, primarly how you define "perceptible to the human eye". The parameter measuring the movement of the stars is called proper motion, and it depends mainly on the dynamics of the star and the distance. Each star has it own proper motion, so it's difficult to give a general answer to your question. There are several examples ...


4

It was fairly poor. Wikipedia has a list of large, potentially hazardous asteroids, and 9 on that list were known before 1998: 2201 Olijato 1620 Geographos 4183 Cuno 1981 Midas 3122 Florence 3200 Phaethon 4486 Mithra 4197 Toutatis (4953) 1990 MU The largest of these, 3200 Phaethon, is 5.8 km, so considerably smaller than the Yucatan impactor, but big ...


Only top voted, non community-wiki answers of a minimum length are eligible