The theory of relativity explains the relationship between space and time through two theories. Special relativity applies to physical phenomena without gravity, while general relativity explains gravitation and its relation to other forces. Einstein's mass-energy equivalence formula, E=mc2, shows that mass and energy are interchangeable. Parallax is used to measure the distance to stars using Earth's orbit as a baseline. A light year is defined as the distance light travels in one year, or about 9 trillion kilometers.

2. THEORY OF
RELATIVITY

3. Special relativity applies to
all physical phenomena in the
absence of gravity. A physical
theory which states the
relationship between space
and time.

4. General relativity
explains the law of
gravitation and its
relation to other
forces of nature.

5. E = mc2, equation in German-
born physicist Albert Einstein's
theory of special relativity that
expresses the fact that mass and
energy are the same physical entity
and can be changed into each
other.

13. Planets in and
beyond the
Solar System

14. One technique astronomers used in
measuring the distance of a star is parallax
method using a baseline of 1 astronomical
unit (AU) which is the average distance
between Earth and the sun, about 93 million
miles (150 million kilometers. They also
measure small angles in arc seconds, which
are tiny fractions of a degree on the night
sky.

15. A light year is defined as the distance that light
can travel in one year. The speed of light is
300,000 km/s. To calculate how far light travels
in one year, first calculate how many seconds
there are in a year: 60 s/min x 60 min/hr x 24
hr/d x 365 d/yr = 31,536,000 s/yr. So in one year,
light will travel 31,536,000 s X 300,000 km/s =
9,461 trillion (9,461,000,000,000,000) km. This is
the same distance as traveling around the world
118 million times in 1 year.