Getting the Measure ofGetting the Measure ofthe Universethe Universe

Dr. Martin Hendry

Glasgow University

Ptolemy: 90 – 168 AD

Nicolaus Copernicus1473 – 1543 AD

Retrograde motion of Mars

Earth

Sun

Venus

We can use Pythagoras’ theorem!!

Solar Eclipses Solar Eclipses

Total Eclipse Zone only about 20km across, but sweeps over many countries as the Earth spins

Tycho Brahe:1546 – 1601 AD

Kepler’s laws, published 1609, 1619

Galileo Galilei:1564 – 1642 AD

Galilean Moons: 1610

Isaac Newton:1642 – 1727 AD

The Principia: 1684 - 1686

Nowadays we can use radar:Nowadays we can use radar:

Distance = Speed x TimeDistance = Speed x Time

Nowadays we can use radar:Nowadays we can use radar:

Distance = Speed x TimeDistance = Speed x Time

But what is the speed of light?…

The ElectromagneticSpectrum

Light wavesLight waves

Wavelength

Frequency = No of waves producedper second

Light wavesLight waves

Wavelength

Frequency = 2450 MHz = 2450 Million waves

per second

Light wavesLight waves

Wavelength

Speed = 2450 million wavelength

Light wavesLight waves

Light travels 300,000 km everyLight travels 300,000 km everySecond……Second……

…………That’s about 10 million, That’s about 10 million, million kilometres every year!!!million kilometres every year!!!

The stars are VERY far away. The nearest star (after the Sun) is about 40 million million km from the Earth. It takes light more than 4 years to travel this distance.

If the distance from the Earth to the Sun were the width of this screen, the next nearest star would be in Rome.

Measuring Astronomical Distances: Parallax

Measuring Astronomical Distances: Parallax

Even the nearest star shows a parallax shift of only 1/2000th the width of the full Moon

SpectroscopySpectroscopy

What can we learn from spectra?

Absorption

e -

e -

Emission

e - e -

Wobbly stars: the key to finding extra-solar planets

Planets and stars orbit their centre of mass

Planets are too faint to see directly

- so stars wobble

but

The Sun’s “wobble”, due to Jupiter, seen from 30 light years away = width of a 10p coin in Madrid

Doppler Shift Doppler Shift

Star

Laboratory

51 Peg – the first new planet

So we can use spectra to tell what stars are made of…..

……If those stars have planets, we can also use spectra to find them, and tell what gases are in their atmospheres!