Hot answers tagged

90

Impacting solar system objects would have relative closing speeds from around 11 to 72 km/s. We could take the optimal case that the asteroid approaches whilst fully lit by the Sun (which I think precludes the minimum and maximum speed in the range quoted above) and then scale from another similar body - say the asteroid Vesta. This has a diameter of around ...


80

I think it's maybe not the case that there was a moment when the astronomy community conclusively rejected the ball-of-fire hypothesis; astronomers simply accumulated more and more evidence against it. If you want to put a rough date on it, you could put your finger somewhere in the middle of the 19th century, as by then, other ideas had taken hold. Back in ...


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 ...


40

The answer is ironic: Without good instruments, there is no evidence. The people who thought that the Sun went around the Earth were perfectly correct as far as the actual evidence went until the early 1700s and mid-1800s when two lines of evidence opened up that showed that the Earth moved. Aberration of Starlight Wikipedia has a correct but over-...


33

Ole Rømer did not measure a change in the frequency of light. He measured an apparent change in the orbital period of Io, one of Jupiter's moons. The orbit of Io can be measured very accurately by observing when it enters or leaves the shadow of Jupiter. When the Earth is moving away from Jupiter, Rømer noted that the orbit of Io appeared to be very ...


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. ...


30

Kepler's account of how the third law came to be is as follows (Caspar p.286; emphasis mine): On the 8th of March of this year 1618, if exact information about the time is desired, it appeared in my head. But I was unlucky when I inserted it into the calculation, and rejected it as false. Finally, on May 15, it came again and with a new onset conquered ...


30

Hipparchus, not Kepler Kepler got the conic sections right, and Newton gave us the mechanics. But the question is about when people knew that the Earth was closer to Sol in one part of the year than others, and Hipparchus knew that, even though he wasn't too hot on the values of the orbital radii. Hipparchus' version of the eccentric model had Sol's (...


29

You make a great point. The reason behind the discrepancy between the dates is due to a complicated history behind it. The calendar is based on the calendar created by ancient Romans, which is based on one Moon cycle. One lunar cycle is 29.53 days. www.universetoday.com/20620/lunar-year/ which does not evenly divide into the 365.25 days of ...


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 ...


25

Kepler's third law is trivial (in my opinion) compared to his first law. I am quite impressed that he was able to deduce that the orbits were ellipses. To get that, he had to go back and forth plotting Mars' direction from Earth and Earth's direction from Mars. He knew the length of both planets' years, so observations taken one Mars year apart would differ ...


25

The book The Transits of Venus, by Sheehan and Westfall, describes how Aristarchus used Hipparchus' calculation of the Earth-Moon distance, who in turn used Eratosthenes' calculation of the Earth's circumference, to calculate the Earth-Sun distance. Aristarchus of Samos was the first to seriously calculate the distance to the Sun, using geometry. When the ...


23

Olber's Paradox was created at a time before the idea of a finite universe was accepted. (It was thought of in the 1600's). In order to resolve Olber's Paradox, you have to introduce the idea that either the universe had a beginning or it is of finite size. (Note: the solution does not require an expanding universe). So, at the time, it was a paradox. ...


23

Johannes Kepler Wikipedia: "However, childhood smallpox left him with weak vision and crippled hands, limiting his ability in the observational aspects of astronomy." He made great use of Tycho Brahes great systematic observations in his theoretical work. He did not need exceptional eyesight for his developments in optics and telescopes.


23

Scientists figured that the sun couldn't be a ball of coal during the industrial age, because given the mass of the Sun, all the coal would have burned out before humans appeared on Earth. But we didn't know what else could be going on. In 1904, Rutherford suggested radioactive decay as a possible process that could account for the Sun's energy. But it is ...


20

1. Ancient cultures observed the sky Night skies are naturally dark and there was no light-pollution in ancient times. So if weather permits, you can easily see a lot of stars. No need to tell about the Sun and the Moon. Ancient people had good reasons to study the night skies. In many cultures and civilizations, stars (and also the Sun and the Moon) where ...


20

Following a reference to Darley et al., ApJ 746, 61 (2012) from your Wikipedia link gives a (very technical) discussion of nova progenitors, including distinctions between nova systems where the secondary stars are main sequence or supergiant stars, and distinctions among white dwarfs with different chemistries. The first sentence of that paper is A ...


19

You cannot prove that the Earth orbits the Sun rather than vice versa because this goes very much against the grain of all frames of reference being equally valid (but some make a lot more sense than others). For example, it makes much more sense to use an Earth-centered, Earth-fixed point of view rather than a non-rotating geocentric, heliocentric, ...


19

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, ...


18

Yes, and lunar occultations have proved useful in several cases. Hazard et al. 1963 used a lunar occultation to produce a high-resolution brightness profile of the now well-studied radio quasar 3C 273. Scheuer 1965 goes into a little bit of detail on general computations. A slightly different tack was taken by Vedantham et al. 2015. They were attempting to ...


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

The authors assume a geocentric universe (first thing that is wrong). They then assume that the planet Mars has the same apparent diameter as a globe 30 yojana in diameter (about 150 miles) in the same orbit as the moon, from a perspective at the centre of the Earth. This is just stated, and appears to be supposition. It is an incorrect figure. It is ...


14

To first order, the relative abundances of the heavier elements to iron (for instance) are constant. So the metal content of a star is shorthand for the content of any element heavier than He. (NB: we now know this is not true in many circumstances and elements can be grouped by synthesis process - for example we can talk about "alpha elements" - O,...


14

I'm pretty certain it is the 200inch Hale telescope at Palomar Observatory. There is an image galley of the 200 inch telescope. According to Richard Preston's book First Light: The Search For The Edge Of The Universe, Einstein attended the completion of the telescope truss, yoke and horseshoe bearing at Westinghouse Electric and Manufacturing Company in ...


14

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 ...


14

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 ...


13

Despite of the explanation of your question, it is a valid question to ask why planets all have Roman names. First of all, the Romans could, like the Greeks and Sumerians, could only see Mercury, Venus, Mars, Jupiter and Saturn. These planets can be seen with the naked eye. However, the fact that the Romans could see these planets, didn't give all the ...


13

From wikipedia, the rings of uranus The definitive discovery of the Uranian Rings [...] use[d] the occultation of the star SAO 158687[...] The star SAO 158687, also cataloged as HD 128598 is a magnitude 8.7, orange dwarf star in Libra. There seems to be nothing special about the star, except that one day, a planet passed in front of it.


12

If you start with the idea that the planets, the sun, the moon and the earth are all bodies that all move through space, exclude the apparently fixed stars, and then see what evidence there is as to how they move relative to each other, then in that context there is some evidence to be found in naked-eye astronomy aided by navigational instruments available ...


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