- published: 09 Oct 2014
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A light-year, also light year or lightyear (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.
The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data.
1 light-year = 9460730472580800 metres (exactly)
The figures above are based on a Julian year (not Gregorian year) of exactly 365.25 days (each of exactly 86400 SI seconds, totalling 31557600 seconds) and a defined speed of light of 299792458 m/s, both included in the IAU (1976) System of Astronomical Constants, used since 1984.
Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983. The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of 9.460530×1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000.
Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has a wavelength in the range of about 380 nanometres to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths.
Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second (about 300,000 kilometers per second), is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR) is experimentally found to move at exactly this same speed in vacuum.
In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.
A year (from Old English gēar) is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic.
In astronomy, the Julian year is a unit of time, defined as 365.25 days of 86400 SI seconds each.
There is no universally accepted symbol for the year as a unit of time. The International System of Units does not propose one. A common abbreviation in international use is a (for Latin annus), in English also y or yr.
Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, hours of daylight, and consequently vegetation and fertility. In temperate and subpolar regions, generally four seasons are recognized: spring, summer, autumn and winter, astronomically marked by the Sun reaching the points of equinox and solstice, although the climatic seasons lag behind their astronomical markers. In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season.
I go back on what I said,
I take back the things I did.
I know, I've been unfair to you.
Oh, I know, I've been unfair to you.
It's like, I'm a staple in the middle of the page,
Where the ink won't run, and the ink won't stay.
I know, I've been unfair to you
Oh, I know I'm never there for you.
You and me are light years apart,
You and me are light years,
You and me are light years apart,
You and me are light years.
It's not up to me, oh
It's down to you.
I can change if you can too.
We got caught in the middle,
Caught out here,
We got caught in the middle,
With every word, every time.
Our lives break up and we never compromise,
These words break down cause we never took the time.
You and me are light years apart,
You and me are light years,
You and me are light years apart,
You and me are light years.
It's not up to me, oh
It's down to you.
I can change if you can too.
We got caught in the middle,
Caught out here,
We got caught in the middle,
With every word, every time.
I go back on what I said,
I take back the things I did.
I know, I'm never there for you.
Oh, I know, I'm never there for you.
You and me are light years apart,
You and me are light years,
You and me are light years apart,
You and me are light years.
It's not up to me, oh
It's down to you.
I can change if you can too.
We got caught in the middle,
Caught out here,
We got caught in the middle,
With every word...
It's not up to me, oh
It's down to you.
I can change if you can too.
We got caught in the middle,
Caught out here,
We got caught in the middle,
With every word, every time.
You and me are light years,
You and me are light years apart,
You and me are light years.
You and me are light years,
You and me are light years apart,