Law of amount of interference between different Sun rays that fall on a distinct point on Earth? The double slit experiment shows that rays comming from different locations can cancel each other when touching a point on a screen if they are out of fase because of their wave properties. Is the same happening with rays comming from different points on the Sun that hit a distinct point on Earth. What would be the ratio between their intensity if they all added up when hitting that point and the actual measured intensity?
 A: The light from different points on the Sun is incoherent. There is no fixed phase relationship between the light rays, so they fall in and out of phase randomly and rapidly, so no fixed interference pattern is produced.
In the two slit experiment both slits are illuminated by the same small light source and therefore act approximately as coherent light sources with a fixed phase relationship. That is the difference.
A: In order to obtain interference effect some conditions have to be met.
Here is a phenomenon that we can see without a specific experimental setup: it occurs in plain daylight: the color effects when there is a very thin film of a material that will partialy reflect and partially transmit. This is referred to as thin film interference
A specific example of that is when some gasoline has spilled, and the gasoline is spreading out over a puddle of water. When the gasoline layer is thin enough to be a small multiple of the wavelength of visible light then there are color effects. The spectrum of light that our eyes are sensitive to is from about 400 nm wavelength to about 800 nm wavelength. The light from the Sun reaching the layer of gasoline on the water is partially reflected at the air-gasoline interface, and partially transmitted down. What is transmitted down tends to be reflected at the gasoline-water interface. There is a relatively low probability of being reflected on the way back up. As a result: light that reaches your eye can have traveled along two different paths, with just a slight difference in length.
Depending on the wavelength of the light that particular pathlength difference gives causes the phase of the traveled-along-different paths light to come out either in phase or out of phase. So for the light that reaches your eye from a particular spot of the gasoline: some colors of the daylight spectrum will be suppressed, as pathlength difference can make some of the light ending up out of phase.
So the thinness of the layer of gasoline is a crucial factor here. Because of that thinness there are really just two paths for the light to reach your eye. (Because of the constraint of the thinness of the layer it doesn't particularly matter what direction the incident light was coming from.)
As soon as you get away from the tight constraint such as in the case of a thin layer the odds of getting an interference effect are very low.
We don't see color effects when light is interacting with a window pane. The glass of a window pane does have some ratio of reflection and transmission, but the glass is so thick that there are many, many different paths for the light to reach your eye, hence no interference effects.
The Sun is a large source, the opposite of a point source. That gives multiple paths for light to reach your eye, hence no interference effects.



[Later edit to address a question in the comment section]
The question in the comment section is about more than one laser source illuminating a small spot.
I will get to that (if impatient: scroll down), first I want to discuss several examples, illustrating how the conditions for interference effect to occur can be met.
There is the case of thin layers, such as a thin layer of gasoline on a puddle of water, the thin film of a soap bubble, anti-reflective coating applied on glass.
In all of those instances of a thin layer you get color effects that arise from interference effect. There are no constraints on the source of the light: you get visible-to-the-naked-eye effect with plain daylight.
Newton's rings
The next item I want to discuss is Newton's rings. (From here on I will assume you have absorbed the content of the wikipedia article about Newton's rings.)
Again, Newton's rings is an interference effect that is obtained with plain daylight.
When Newton's rings are obtained with plain daylight there are chromatic effects.
A Newton's rings setup that is illuminated with mono-chromatic light is more vivid, because of with mono-chromatic light any chromatic effect is below the detection level of the naked eye.
The light from a Sodium lamp is not exhausitively mono-chromatic, but it is sufficiently monochromatic any chromatic effect is  below naked eye detection level.
Note that a sodium lamp is not laser source. However, compared to plain daylight the light of a Sodium lamp is to an extent constrained; constrained to being a mono-chromatic source. That constraint of the source allows Newton's rings to be clearly visible over a larger area as compared to illumination with plain daylight.
An additional constraint that can be applied to a light source is to illuminate a screen with only pin-hole in it, and use only the light that emanates from that pinhole. Of course you have very little luminosity left that way, but that spatial constraint has the effect that the light is down to very little freedom of different pathlength. That additional constraint on the source of the light allows for richer interference effect to be obtained.
Dennis Gabor created the first Holograms, and he used Mercury arc light, passed through a filter that allowed only a narrow frequency band of light to pass. The arc light was a sufficiently small source to act as a point source of light
An essential property of using a laser as light source is that the emission of that light is spatially constrained. In effect the light is as spatially constrained as light that emanates from a pinhole. As mentioned in the preceding paragraph: light that emanates from a pinhole is down to very little freedom of different pathlength.
As soon as laser sources became available the creation of holograms shifted to using laser light as light source. With the light source that Dennis Gabor had used it was hard to get to the required luminosity. Laser sources have the luminosity, and the required spatial constraint.
More than one laser source
There is the following property of emitters of laser light: while at any point in time all of the emitted light is in phase, over time there is drift of that phase. How quickly or how slow that drift occurs is referred to as 'temporal coherence'. Rapid drift: low temporal coherence.
There is also a random drift in the frequency of emitted light. It is possible to make the fluttering of that random drift go slower, but that requires a more intricate setup, which is of course more expensive.
If you have a single laser source, and you use a beam splitter to split the light in two beams, and you recombine those beams, you obtain interference effect because the two beams, having originated from a single source, have the property of temporal coherence. Also, the random drift of frequency has no discernible effect on the inferference pattern, because the shape of the interference pattern changes very little with change of the frequency of the light. The interference pattern shows pattern of being in phase and out of phase, which is a function of difference of pathlength.
When you use two or more distinct laser sources you don't get any of that temporal coherence from one laser to another.
I want to mention/emphasize here that interference effect does not alter the total luminosity received at the detector. Interference effect alters the distribution of the luminosity.
In order to measure luminosity of incoming light a detector must aggregate over time. If you would use the equivalent of a very, very short shutter time you are down to very low luminosity.
The temporal counterpart of a spatial interference pattern is a beat frequency. If you have two sources of laser light then if you would be able to detect with very high temporal resolution I suppose you would detect a drifting beat frequency. (Drifting because the frequencies of the individual laser sources are subject to random drift.) But realistically you have to aggregate over time, and over time any beat frequency effect averages out.
With any temporal effect averaged out what you end up with is the total luminosity. Again, interference effect does not alter total luminosity, only the distribution of luminosity. A beat frequency is a periodic distribution of luminosity over time. Averaged over time you end up with the sum luminosity of the two individual laser sources.

So: specifically to your question in the comment section:
Assuming a luminosity detector is used that averages luminosity over time (which all practical detectors do): shining the light of two laser sources onto the same detector will result in the detector reporting the sum of the luminosities of the individual laser sources.
A: Interference unfortunately is one of the most misunderstood concepts in physics. 2 photons never cancel, that would be a violation of conservation of energy. In 1801 Sir Thomas Young showed the pattern, it looked similar to how water behaved and the word "interference" was used. If you take a Quantum Optics course you will realize in the DSE that there are no photons in the dark areas of the pattern, all the photons go to the bright areas.
