# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: guest39538 on 04/06/2016 08:26:52

Title: How do we know it takes ''light'' approx 8 minutes to reach us from the sun?
Post by: guest39538 on 04/06/2016 08:26:52
Hello, I have been giving some though to the understanding of the light that ''travels'' from the Sun to the Earth passing through space and any other ''things'' that may occupy ''empty'' space.
A time is devised by a speed over distance, in the process of recording a time something takes we have to have two point .  We then ''time'' the journey.

So if we define the Sun is (a) and the Earth is (b)  and I was to time the ''light'' from what exactly am I timing of the journey?

I can see the Sun and I can also see the Earth but I do not see anything travelling from the Sun to ''time'', the relative view from and likewise is ''gin-clear'' and I do not observe anything to ''time'' or do I see any travelling of ''light'' , I only observe a relative ''empty space'' between point sources.
Title: Re: How do we know it takes ''light'' approx 8 minutes to reach us from the sun?
Post by: alancalverd on 04/06/2016 09:29:00
If you post a letter or an email, you don't see it travelling, but you can measure the time it took to get there. If you know how fast a horse can run, you can calculate when it will finish a 2-mile race (please gamble responsibly).
Title: Re: How do we know it takes ''light'' approx 8 minutes to reach us from the sun?
Post by: guest39538 on 04/06/2016 10:11:07
If you post a letter or an email, you don't see it travelling, but you can measure the time it took to get there. If you know how fast a horse can run, you can calculate when it will finish a 2-mile race (please gamble responsibly).

Yes I would agree , but you can only measure the time the letter took when you know what time it was sent. We do not know what ''time'' a ''photon'' leaves the Sun, and it is very factual we do not see individual ''photons''  leaving the Sun because space is ''gin-clear'' relative to observation.
I understand you can create an experiment and measure the time it takes for the 'tip' of the light to reach a destination and then make your assumptions based on that, but how do we know  that once the ''tip'' as arrived it does not ''stop'' or is delayed in some way?
A rate of flow is different to the rate of a single ''particle''?
Flows create pressure where as a particle would create force?

I have to question the existence of a photon Alan, a drip from a tap is man made , the naturalness of the tap water is to flow and not to be a drip. If I quickly turn on and off a switch I get a ''drip'' of electricity, if I leave the switch ''open'' I get a flow?

Title: Re: How do we know it takes ''light'' approx 8 minutes to reach us from the sun?
Post by: Colin2B on 04/06/2016 15:27:52
If I quickly turn on and off a switch I get a ''drip'' of electricity
That drip causes the light to switch on & off.
One of the ways of measuring the speed of light was to use planets or moons passing in front of others - switching the light off then on. Look up Ole Romer and see how he did it.
Title: Re: How do we know it takes ''light'' approx 8 minutes to reach us from the sun?
Post by: evan_au on 05/06/2016 00:08:36
Quote from: TheBox
I can see the Sun and I can also see the Earth but I do not see anything travelling from the Sun to ''time''
To time something, you need to know the time it left, and the time it arrived.

Unfortunately, it takes an enormous amount of rocket fuel to deliver scientific instruments to the surface of the Sun - and they would vaporize long before they got there. So measuring the departure time is tricky!

However, in 2006 a pair of Sun-observation satellites (STEREO-A and STEREO-B) were launched into orbits where one drifted ahead of Earth, and the other drifted behind the Earth. Because these were not actually approaching the Sun, it only took a gravitational slingshot around the Moon to get them into their desired orbits.

These satellites need to have a good idea of their position in 3-dimensional space, so they can point their antennas at Earth. These satellites also need to have an accurate idea of the time on Earth, so they can transmit their results at the right time. Their radio transmissions need to arrive at Earth at the right time that the deep-space tracking antennas are looking for their signal, and pointing in the right direction.

In 2015, STEREO-A passed behind the Sun. While it was on a fairly close line-of-sight to the Sun, the microwave signals would have travelled about twice the Earth-Sun distance, and taken twice the Earth-Sun delay to reach the Earth.

This is a case where we have been able to place scientific instruments in a location that lets us measure the Earth-Sun delay of the speed of light fairly directly.
See: https://en.wikipedia.org/wiki/STEREO
Title: Re: How do we know it takes ''light'' approx 8 minutes to reach us from the sun?
Post by: guest39538 on 05/06/2016 08:12:35
Quote from: TheBox
I can see the Sun and I can also see the Earth but I do not see anything travelling from the Sun to ''time''
To time something, you need to know the time it left, and the time it arrived.

Unfortunately, it takes an enormous amount of rocket fuel to deliver scientific instruments to the surface of the Sun - and they would vaporize long before they got there. So measuring the departure time is tricky!

However, in 2006 a pair of Sun-observation satellites (STEREO-A and STEREO-B) were launched into orbits where one drifted ahead of Earth, and the other drifted behind the Earth. Because these were not actually approaching the Sun, it only took a gravitational slingshot around the Moon to get them into their desired orbits.

These satellites need to have a good idea of their position in 3-dimensional space, so they can point their antennas at Earth. These satellites also need to have an accurate idea of the time on Earth, so they can transmit their results at the right time. Their radio transmissions need to arrive at Earth at the right time that the deep-space tracking antennas are looking for their signal, and pointing in the right direction.

In 2015, STEREO-A passed behind the Sun. While it was on a fairly close line-of-sight to the Sun, the microwave signals would have travelled about twice the Earth-Sun distance, and taken twice the Earth-Sun delay to reach the Earth.

This is a case where we have been able to place scientific instruments in a location that lets us measure the Earth-Sun delay of the speed of light fairly directly.
See: https://en.wikipedia.org/wiki/STEREO

Ok, I would accept that about speed, but I do not feel that in any way answers me in full so I may return with further questions on this matter.