Wikipedia:Reference desk/Archives/Science/2020 April 18

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April 18

Is sidereal month constant?

According to what I see here: "the time interval between two consecutive identical phases of the moon, for example, the time between consecutive new moons. The duration of the synodic month is not constant; the average length is 29.530588 mean solar days, and deviations range up to 13 hours". Is the duration of the synodic month is not constant because the the duration of the sidereal month isn't constant too, or the synodic month is the only one that isn't constant (while the the sidereal is)?

What's the reason for deviation of up to 13 hours in the synodic month?

What's the reason for deviation of up to 13 hours in the synodic month? and where I can find tables for this deviation in details with example of 12 months comparison? It's really interesting to see current examples. Thank you ThePupil (talk) 02:17, 18 April 2020 (UTC)

The sidereal month (also defined on the same site as your link in your previous query) varies with the time of year, because it's measured in relation to the background stars as seen from Earth, which drift around a full 360° over the course of one year (i.e. one orbit of the Earth around the Sun).

If the Earth's orbit around the Sun were circular, this drift would be constant, so the Sidereal month would be (close to) constant. However, the Earth's orbit is actually elliptical: the Earth is closest to the Sun in January and furthest away six months later, so it is moving fastest in January and slowest in July. Consequently the rate of drift of the background stars also varies, and in January (for example), the Moon has to travel a bit further to regain the same position with respect to them.

This is the main reason for the deviations, but there are other longer term variations in both the Moon's orbit around the Earth (see also Lunar theory) and the Earth's around the Sun, which also contribute small amounts.

In my experience, astronomers calculate the deviations using complex formulae when they need to rather than looking them up in tables, but its some decades since I was much involved in this stuff, so I don't know what current references might exist. {The poster formerly known as 87.81.230.195} 90.203.117.240 (talk) 03:20, 18 April 2020 (UTC)

charged particles emit virtual photons at each other? What wavelength/energy?

What kind of function of charge amounts and separation distance is it? Is an integer number of half wavelengths equal to the separation? Also what kind of wavelength/energy would be the photons between quarks inside of a proton?Rich (talk) 07:21, 18 April 2020 (UTC)

The interaction between quarks is mediated by gluons, not photons.  --Lambiam 08:12, 18 April 2020 (UTC)

They're charged particles - there will be an electrical interaction, even if it's not the dominant one. Someguy1221 (talk) 13:52, 18 April 2020 (UTC)

Elaboration: quarks have electric charge as well as color charge (associated with the strong interaction). They interact with each other electromagnetically, but when they are very close to each other, the strong force is much more powerful. --47.146.63.87 (talk) 22:36, 19 April 2020 (UTC)

Nothing is emitted. What is going on here is that a model that describes the true physical situation where the particles couple to photons cannot be treated exactly. We then consider a simplified model where the interaction between the particles and photons is zero, and we then perform an expansion around this zero interaction strength to get to a series expansion in powers of the interaction strength. what then happens is that the terms in this series expansion can be represented diagrammatically (Feynman diagrams) that look like physical processes where photons are emitted and absorbed. But these are not real physical processes. In these computations the so-called virtual photons can have energies and momenta that are not consistent with zero mass, they can have any mass, even an imaginary mass, because you have to integrate over the energies and momenta independently. Count Iblis (talk) 23:23, 18 April 2020 (UTC)

thanks!Rich (talk) 01:07, 19 April 2020 (UTC)

You might also find this helpful, again from Prof. Matt Strassler, written for a general audience. --47.146.63.87 (talk) 22:36, 19 April 2020 (UTC)