Either the object creates new light or it reflects light that already existed. Objects that create light tend to also reflect ambient light, so that they tend to be the brightest objects around.
Examples include campfires, light bulbs, candle flames, and computer screens. In terms of astronomical bodies, stars are the main objects that create significant amounts of visible light, and therefore are some of the brightest objects in the universe.
If a planet somehow became large enough to initiate nuclear fusion and begin glowing, it would no longer be a planet. It would be a star. Since planets and moons do not emit light, the only reason we can see them is because they reflect light from some other source. The strongest source of light in our solar system is the sun, so usually we see planets and moons because they are reflecting sunlight.
The amount of sunlight incident on a moon or planet that gets reflected depends on the materials in its surface and atmosphere as well as its surface roughness. Snow, rough ice, and clouds are highly reflective. Most types of rock are not. Therefore, a planet that is covered with clouds, such as Earth or Venus, is generally brighter than a rocky moon or planet that has no atmosphere.
There are two main types of reflectivity: specular reflectivity and diffuse reflectivity. Specular reflectivity measures how much of the incoming light gets reflected by the object in the direction given by the mirror angle. The other half of the surface faces away from the sun and is in shadow. The moon is at its brightest when it is degrees away from the sun from our perspective picture the sun, Earth and moon in a straight line. At this time, the full half of the moon's surface facing the sun is illuminated and is visible from Earth.
This is what's known as a full moon. Within that lunar cycle, there are eight distinct phases. Most days, the Moon is above the horizon for 12 hours. The best chances to see the Moon and the Sun in the same sky are right after a full moon phase until just before a new moon.
Major calendars often include the phases of the Moon each month. Make a resolution for you and your family to spend time studying the skies this year. A moonlit stroll is starkly different from a walk in the sunshine. Moonlight's dark, spooky quality contrasts with the clarity of sunlight. And while it may not grow hair on your face, we can't help but notice the blacker shadows, blurred details, and lack of color in the landscape on a moonlit night. We peer into the gloom straining to see what's there, but lacking the usual visual cues, we're liable to fill in the darkness with our imagination.
Is that Bigfoot up ahead or the neighbor's garbage can? For these reasons, it's often used in scary movies where exaggerated contrasts under studio moonlight create an unfamiliar landscape where anything can happen, including romance. For many of us, both moonless and moon-full nights simply aren't bright enough to stimulate the retina's cone cells responsible for detail and color vision.
At low light levels, a different group of light-sensing cells called rods go to work. Rod cells are seriously sensitive, able to detect a single photon of light, but they lack the ability to see color and detail. On a moonless night away from city lights, the nightscape is painted in shades of gray, charcoal black, and bony whites.
Things aren't so simple when the Moon is out, something you can see for yourself this week as the gibbous Moon waxes to full by Sunday night. Moonlight is essentially sunlight reflected from the Moon's surface. Makes sense. Light striking the Moon's ubiquitous pulverized dust is actually somewhat redder than sunlight, but we don't see this with our eyes because it's offset by all the other colors contained in moonlight. A high-riding Moon gleams with silvery-white radiance.
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