How Far?

Measuring Distance in the Universe

How?

• Gian Domenico Cassini, in 1672, used a technique called parallax to measure the distance to Mars.

Cassini figured it out!

• The Sun is about 93 million miles from

• Earth. As Earth and Mars never come closer than 35 million miles to each other.

• Saturn, the most distant planet known when Cassini was alive, is around 900 million miles away.

How?

• Parallax!

What is Parallax?

• Try this out:– Hold 1 finger out at

arm’s length

– Close one eye, and then the other

– Notice how your finger “jumps?

Measuring Stars

• Stars are billions of light years away – winking won’t help

• Our orbit is 186 million miles wide (93 million + 93 million)– Star A and B are the

same star, as they appear 6 months apart

How to:

• The parallax angle p is illustrated in the above figure.

• We can determine they adjacent (d) using trigonometry.

Trigonometry

• Our adjacent line (d in the prior picture) is the distance from Earth’s Orbit (BC) to the star (A)

• Adjacent length= opposite length tangent angle A

Trigonometry

• Measure the angle of the star to our central point of orbit

• Simple calculation• Even the Hubble Telescope limited to 1000 LY

Earth’s Orbit

Parsec

• The method of parallax gives rise to a natural distance unit that astronomers call the parsec. – defined as the distance at

which a star would have a parallax angle p equal to one second of arc.

– using trigonometry distance equal to 206,265 astronomical units or 3.26 LY

• AU is the average distance of Earth to Sun

Further away?

• The universe is expanding

• Galaxies furthest away are moving away faster than galaxies closer to us

• We can us Doppler Effect

Visible Spectrum

Doppler

• Doppler

What is Red Shift?

• As light moves away, the waves stretch out• Longer waves are shifted towards the red• As light moves towards us, the waves condense• Shorter waves are shifted towards the blue

Spectral Signature

• Each element has a unique spectral signature• Two elements combine