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Getting the Measure of
the Universe
Dr. Martin Hendry
Glasgow University
Ptolemy: 90 – 168 AD
Nicolaus Copernicus
1473 – 1543 AD
Retrograde motion of Mars
Sun
Earth
Venus
We can use Pythagoras’ theorem!!
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:
Distance = Speed x Time
Nowadays we can use radar:
Distance = Speed x Time
But what is the speed of light?…
The Electromagnetic
Spectrum
Light waves
Light waves
Wavelength
Frequency = No of waves produced
per second
Light waves
Wavelength
Frequency = 2450 MHz
= 2450 Million waves
per second
Light waves
Wavelength

Speed = 2450 million

wavelength
Light travels 300,000 km every
Second……
……That’s about 10 million,
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
Spectroscopy
What can we learn from spectra?
Absorption
ee-
Emission
ee-
Wobbly stars: the key to finding
extra-solar planets
Planets are too faint to see directly
but
Planets and stars orbit their centre of mass
- so stars wobble
The Sun’s “wobble”, due to Jupiter, seen from 30 light
years away = width of a 10p coin in Madrid
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!