light year

a lightyear is a unit of interstellar space measurement. One
l.y. (lightyear) is how far light will travel in one year. How far
is that? Well if light travels 299,800 kilometers in 1 second
and there are 31,560,000 seconds in a year that means in one
year light can travel....
9,461,000,000,000 km!
A light year is a way of measuring distance. That doesn't
make much sense because "light year" contains the word
"year," which is normally a unit of time. Even so, light years
measure distance.
Cool Fact
A light nanosecond -- the distance light can travel in a
billionth of a second -- is about 1 foot (about 30 cm). Radar
uses this fact to measure how far away something like an
airplane is. A radar antenna sends out a short radio pulse and
then waits for it to echo off an airplane or other target. While
it's waiting, it counts the number of nanoseconds that pass.
Radio waves travel at the speed of light, so the number of
nanoseconds divided by 2 tells the radar unit how far away the
object is!
Using a light year as a distance measurement has
another advantage -- it helps you determine age.
Let's say that a star is 1 million light years away.
The light from that star has traveled at the speed
of light to reach us. Therefore, it has taken the
star's light 1 million years to get here, and the
light we are seeing was created 1 million years
ago. So the star we are seeing is really how the
star looked a million years ago, not how it looks
In the same way, our sun is 8 or so light minutes
away. If the sun were to suddenly explode right now,
we wouldn't know about it for eight minutes because
that is how long it would take for the light of the
explosion to get here
Why would you want such a big unit of distance? Well,
on Earth, a kilometer may be just fine. It is a few
hundred kilometers from New York City to Washington,
DC; it is a few thousand kilometers from California to
In the Universe, the
kilometer is just too
small to be useful. For
example, the distance
to the next nearest big
galaxy, the Andromeda
Galaxy, is 21 quintillion
000,000,000 km. This
is a number so large
that it becomes hard to
write and hard to
interpret. So
astronomers use other
units of distance.
In our solar system, we tend to describe
distances in terms of the Astronomical Unit
(AU). The AU is defined as the average
distance between the Earth and the Sun. It
is approximately 150 million km (93 million
miles). Mercury can be said to be about 1/3
of an AU from the Sun and Pluto averages
about 40 AU from the Sun. The AU,
however, is not big enough of a unit when
we start talking about distances to objects
outside our solar system.
For distances to other parts of the Milky Way Galaxy (or
even further), astronomers use units of the light-year or
the parsec .
1 parsec (pc) = distance d
when angle is 1 arcsecond =
3.086 1013 km = 3.26 ly
The Andromeda Galaxy is 2.3 million light-years away.
The Milky Way Galaxy is about 150,000 light-years across
The Crab supernova remnant is about 4,000 light-years away.
Stellar parallax
Parallax is the apparent displacement of a foreground object relative
to the background as the observer’s location changes.
The closer the object is the
greater the parallax so small
parallax implies large distance,
large parallax implies small
We can convert baselines and parallaxes into distances.
To compute the planet’s distance;
Distance = baseline x (360o/2p)/parallax
360o/ 2p = 57.3o or 1 radian
Parallax is the angle between two points of the baseline
However at extreme distances
the angle of Parallax becomes
infinitesimally small and hence
Angular Diameter:
By measuring the object’s angular diameter, we can
compute its size.
Diameter = distance x angular diameter/57.3O
Example – The moon’s angular diameter is measured to
about 31 arc minutes – a little over half a degree. By
putting this into the equation we get a diameter of about
Astronomy involves thinking about angles--often very
tiny, tiny angles. A circle is divided into 360 degrees
(or 360°). Each degree is divided into 60 arcminutes
(or 60'). Each arcminute can be divided into 60
arcseconds (or 60"). Therefore there are 360x60x60
= 1,296,000 ~ 1.3 x 106 arcminutes in a full circle.
Distances in the sky are measured in degrees. The degrees are originally
taken from the degrees around a circle. It is easy for star watchers to
measure degrees with their hands. To use your hand as a measuring
tool, you need to hold your hand in front of you at an arm's length.
Since we know the Earth has to rotate completely in 24
hours, and one entire rotation means circling 360°, the rate
in degrees that the Earth rotates per hour is:
360 degrees 15 degrees
Rotation rate = ------------- = -----------24 hour
Or 15° per hour