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Hello:
It's very sad that most people don't know even the fundamentals of space. Unfortunately, some of us can't see beyond our own problems and our surroundings. As a professional, I sometimes wonder just how many of 'our problems' can be solved, just by taking a long hard look at our universe. Once we start understanding the 'complexities' of the same, we are using our brains for 'better' things in this world.
We are lucky to have Internet, that takes us on journeys far beyond the realms of our own existance. I notice that when I 'talk' to many of the people that decide to commit crimes, they always say that they did this, or that, becuase they 'were bored'!
Let's see a little about those 'complexities' that we spoke about earlier; hope you agree with me, but if you don't I respect that.
The Speed of Light:
In empty space (a vacuum), light (and other kinds of electromagnetic radiation) always travels at the same speed, which is therefore usually called the speed of light. That speed of light is by definition exactly equal to 299,792,458 m/s and is usually written c in formulas. In terms of miles, this is equal to 186,283 miles per second (rounded to the nearest whole number).
In ordinary life, speeds add up. If Ann walks 1 km/h faster than Burt and Burt walks 2 km/h faster than Clara, then Ann goes 1 + 2 = 3 km/h faster than Clara. The same does not hold for light (and other kinds of electromagnetic radiation). If Ann, Burt, and Clara walk in the sunshine and measure the speed of the sunlight, then all three of them get the same result, even though they are not all walking equally fast.
If light travels not through empty space but rather through a gas (air) or a transparent fluid or solid, then the speed of the light in that substance is less than the speed of light in empty space. The ratio of the speed of light in empty space and the speed of light in a substance is equal to the index of refraction of that substance. The index of refraction of water is about 1.4 (depending on the color of the light), so the speed of light in water is equal to about c/1.4 which is about 0.7c, which is about 210,000,000 m/s or about 130,000 miles per second.
Here is the speed of light in a number of different units:
value: unit:
1,079,252,849 km/h kilometers per hour
670,618,310 mph miles per hour
299,792,458 m/s metesr per second
299,792.458 km/s kilometers per second
186,282.864 mi/s miles per second
63,241.08 AE/y astronomical units per year
173.14 AE/d astronomical units per day
7.481 earth circumferences per second
7.214 AE/h astronomical units per hour
1 ly/y lightyears per year
0.3066 pc/y parsec per year
0.002004 AE/s astronomical units per second
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To know how long light takes to travel 900 km you need only divide that distance by the speed of light, which is (see above) almost 300,000 km/s. So, 900 km takes light only 900/300,000 = 0.003 seconds to travel (in empty space). 900 km is equivalent to about 560 mi, so you can also calculate the time as 560/186,283 = 0.003 seconds.
Massless particles always travel at the speed of light, so they do not need to be accelerated to that speed. Light (photons) does not need to accelerate from zero to the speed of light, but goes at the speed of light right from the start. Other wave phenomena need no acceleration, either, but go at the appropriate wave speed right from the start.
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How Long Is A Lightyear?
A lightyear is the distance that light travels (through empty space) in one year. A lightyear is not a period, but a distance. The speed of light in empty space is a constant (see above), but one can argue about the length of a year, if you want to be very precise. We'll use the average Julian year of 365.25 days. With this, a lightyear is equal to 299,792,458 * 60 * 60 * 24 * 365.25 = 9,460,730,472,580,800 m or about 9.5 million million kilometers or 5.9 million million miles.
The speed of light can also be combined with shorter units of time such as days and seconds. Here is a list.
unit: length:
lightsecond 299,792,458 m 186,283 mi
lightminute 17,987,547,480 m 11,176,972 mi
lighthour 1,079,252,848,800 m 670,618,310mi
lightday 25,902,068,371,200 m 16,094,839,450 mi
lightyear 9,460,730,472,580,800 m 5,878,640,109,306 mi
Light Phenomena:
Halo:
A halo is a circle of light (or part of such a circle) around the Sun or Moon. "Halo" is also used more generally to refer to any optical phenomenon involving the refraction of light by ice crystals. The most common kind of circular halo surrounds the source of light at a distance of about 22 degrees from the center of the source of light. The most common kind of optical phenomenon involving ice crystals (at least in my experience) are sundogs, which are patches of light (often with the colors of the rainbow in them) at about 22 degrees to the left or right of the Sun.
A halo is caused by sunlight or moonlight being refracted into your direction by small by ice crystals in the atmosphere. If you see a halo, then there must be small ice crystals in that direction in the atmosphere. The shape of the halo is determined by the orientation of the ice crystals in the air, and of course by the presence of ice crystals in the air. If there are no suitable ice crystals in the direction that is appropriate for a halo, then you won't see a halo in that direction.
http://www.anomalynews.com/phorum/read.php?f=6&i=257&t=257
Laters,
Lucy
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