Biology Forums - Study Force

By observation of parallax. Edmund Halley (of comet fame) was the first to come up with the idea to determine the Sun's distance by observing the transit of Venus. A transit of Venus means we are watching Venus pass in front of the Sun. Parallax is the effect whereby the position or direction of an object appears to differ when viewed from different positions. So, Halley figured that two observers on Earth, if in different locations, would see Venus cross the Sun at a slightly different time, because of parallax. The effect is exaggerated in this picture: (https://qph.fs.quoracdn.net/main-qimg-c461e60baffc58f7dbd08eeaca5a6b5f) In this picture (assume Venus is moving counterclockwise about the Sun) we can see that a person a point A on Earth would see the transit as just starting while a person a point B on Earth would see the transit just finishing. Halley knew, based on astronomical observations, that the distance between the Sun and Venus was 0.72 times the distance between the Sun and Earth. So, he figured if we knew the distance between A and B and could calculate theta, we could use trigonometry to determine the distance between Earth and the Sun. It turned out that it wasn't possible to get the required accuracy of measurements this way, so it was determined that rather than taking measurements at one point in time, they could measure the time of each transit, because one perceived transit would be shorter than the other, as depicted in this picture: (https://qph.fs.quoracdn.net/main-qimg-ed65d3111cd699251d335a6c911450bd.webp) Theta could be solved from this by using the Pythagorean Theorem: (https://qph.fs.quoracdn.net/main-qimg-e67efb8d62fc6f7e9ae8f327c2dcf4fb) The radius of the sun is phrased in arcminutes based on observation to be 15.25 arcminutes. The drift rate is just the number of degrees in a circle (360) divided by the number of days to orbit the sun (224 for Venus) and then convert the units from days down to seconds). In 1761, scientists had an opportunity to test this out, and their results were within 2.6% of the actual distance. In 2012 there was another Venus transit. This is a picture of the transit. (https://qph.fs.quoracdn.net/main-qimg-34c933882e400b491473248bfa7685ef-c)

Since 1961, we transmit a radar signal at another planet (or moon or asteroid) and measure how long it takes for the radar echo to return.

Thanks for the detailed explanation which jogs my memory back to my high school days a bit.

So, Halley knew based on astronomical observations with a telescope, in the 1720s or thereabouts?

Were telescopes that good yet?

And how did he know that the distance between the Sun and Venus was .72 times the distance between the Sun and the Earth when he didn't know yet what the latter distance was?

Yes, but wouldn't much of the radar signal be absorbed by a star?  How would any signal return; wouldn't the signal would be very weak?

OK, thanks, that clears it up a lot for me, although there's still a few questions, mainly to do with if telescopic equipment in Halley's time was good enough and if we had radar transmitters powerful enough to send a signal to Pluto and back again.

I'd be interested in creating an explanation video for how the Venus' distance and the sun's was calculated!

How ar eyou going to animate it ? Good article: https://en.wikipedia.org/wiki/1639_transit_of_Venus

That's a great question, I'd really like to know how they calculated the distance known today between Earth and Pluto. I'm certain no radar exists to accomplish this, given that it's too deep in space. I couldn't find a video on YouTube either; I need to create one for the team.

and even if we did, how would the signal get back again intact with the radiation belt in the way?

The signal would most likely dissipate if we're trying to capture it from Earth.; our atmosphere is too protective. That's why it's done in outer space, namely from the space station or a flying saucer/space probe.

Exactly, it wouldn't Most of our measurements have come from probes that skim past the planet

Sorry for late reply, had to build new pc... Or do you mean Venus? We don't have probes that skim past either.

From Wikipedia we learn that there have been numerous unmanned missions to Venus. In fact, ten Soviet probes have achieved a soft landing on the surface, with up to 110 minutes of communication from the surface, all without return. Launch windows occur every 19 months. Furthermore, these days this can be done without any flyby probes. The Arecibo Observatory is a radio telescope in the municipality of Arecibo, Puerto Rico. It's huge, and can send signals that pass through both our planet’s atmosphere and the atmosphere of Venus, where they hit the surface and bounced back to be received by the 100-meter Green Bank Telescope in a process known as bistatic radar.

The strength of the radar signal falls rapidly with distance so for objects within the Solar System we are dealing with very faint reflected signals. That isn't a problem with objects like Venus because with suitable signal processing we can extract the radar reflection from the background noise. The problem with the Sun is that it's a (very) strong emitter of radio waves and this black body background completely swamps the radar reflection.