Okay, so I was watching a movie recently, and there was a night scene. The whole valley could be seen in light, and there were some stars as well, and the actors could be seen, but it just looked like it was night. How do you make a shot look like it was taken at night, especially if there is no signs of civilization around?
The effect is called day-for-night and it's particularly common in old movies.
It's often not very convincing.
Often the best looking effects can come from artificial lighting.
The colour of light is a strong visual cue that it is night time. Strong blues trick the brain and can make a well lit scene look like night. Making artificial lights extra warm (orangy) further enhances the contrast to the cool "moonlight". Lighting obliquely also seems to help as it suggests the scene is darker whilst maintaining details. Here is an example:
This is easy to achieve when you control all the lights (e.g. in a studio) but you can still achieve the night look outdoors. It's important to realise that the sun and moon are both point lightsources, so give out the same quality of light, but in different colours and vastly different quantities. This means you can just underexpose and tinting your images. The one thing you won't be able to do is overpower the sunlight with artificial sources like you can with moonlight.
I have seen scenes that appeared to be at night but when looked at closely, look like they would have been shot during the day and been filtered to a point where they appear like bright night time. The main issue you mentioned is stars. Unless it's cloudy, you won't be able to see any stars in the sky.
As far as image manipulation goes, it's possible to simply shoot as you would in sunlight and post process the image to a point where it mostly appears to be night time.
The key factors in producing such an image are saturation, exposure / noise and blue tint.
At night, our eyes don't see colour until there is sufficient light. The scene will typically be very desaturated to the point where the only colour is the typical "blue" of the moon light.
That being said, the camera does see colour in low light. I guess it depends on which you want to simulate.
Update. My own examples.
I have found that you can create some pretty realistic night shots with a little work in Photoshop.
There are many tutorials that have explain the process. Here is one and a Google Search turns up many available options.
One quick way to change the day into night would be to use the curves adjustment and a solid colored layer set to multiply. This does take a bit of work to get right. As for the stars they are not particular hard to fake or blend in from another photo.
This article has some interesting information.
UPDATE: Here is a bit more detail:
There is a tutorial on this here. Some like the results others do not, you can decide.
Every bit of what I do is very subject and can take a bit of work to make look realistic.
I will normally start by putting down a solid color adjustment layer that is dark dark blue or even black. Then set blend-mode to Multiply and change the opacity down to my liking for the photo.
Next I will switch to my channels and look at each channel to see what has the best contrast. I then will CTRL (PC) or CMD (MAC) Click the layer thumbnail which then makes a selection of that layer. I switch back to my layers and with the Solid Color Adjustment Layer Selected I add a Layer Mask.
I then use a curves adjustment layer and again I work with the settings till I get a darker picture that looks similar to night.
Those are the basic steps, again everything is subjective and it can take some work to get right. The biggest thing I have learned is to experiment and see what happens (obviously keeping duplicate of what works or what I like so I do not lose something)
Hope some of this info can help. =>
EDIT - I recently have been playing with Topaz Labs Adjust and they have a filter called Dark and it creates a nice night time effect.
I'm going to take a stab at my own question, and see if I can get it right...
It seems like the best way to make it appear night in camera is to take a picture at sunrise/sunset, but in the opposite direction from the sun. This will give the minimal light, and the right color. Best is if the sun is down for maybe 20 minutes either way, so the light is a more dark blue color. Or alternatively, shoot on the night of a full moon, so you can use the light from the moon to give the desired affect. Also, it's probably a good idea to slightly underexpose the image.
As far as Stars, well, they just probably won't appear that much anyways. But I imagine that if it is dark enough outside, they will appear, if you have a sensitive enough camera.