5. MODERN PERIOD (1650-present)

5.1 Newton (1642-1727)

Settled once and for all the question of the nature/origin of the force responsible for motions of the heavenly bodies. this was done by invoking a PHYSICAL LAW - something that applies to all bodies, ie. UNIVERSAL.

It was the same force that caused the apple to drop from a tree as which caused the moon to orbit the earth. The force was universal and it was due to the mutual attraction between masses: GRAVITATION.

This force could be expressed mathematically:

$${\rm Force} = \frac{GM_{1}M_{2}}{d^{2}}$$

where M₁ and M₂ are the 2 masses, d the distance between them, and G is a constant. Newton's law of gravitation was successful in reproducing:

• Kepler's 3 laws.
• Orbits of comets (Newton was a close colleague of Halley and predicted that Halley's comet would return 76 years later, which it did).
• Irregularities in the orbit of Uranus (due to presence of another planet, which was later discovered to be Neptune).
• Motions of binary stars (thus showing that Newton's Law of gravitation was valid outside the solar system).

Newton's work unified all the observed behavious of the heavens into one physical law: gravity!

Newton's Cosmology: he argued that his law required the Universe to be infinite in extent, for if the universe was finite, all bodies in it would be pulled together into one body (by gravity), whereas many bodies (stars) were seen. In an infinite universe, matter would be pulled into an infinite number of small condensations.

Newton worried a lot about mutual forces between planets, which he thought must eventually lead to the disintegration of the solar system. To avoid this awful event, Newton envisioned the ``hand of God'' occasionally intervening to reset the clockwork mechanisms of planetary motions. ``The Universe was a mechnical one whose order was maintained by a distant God''.

Summary of `World Views'

Aspect of Model	Ptolemy	Copernicus	Kepler	Newton
Planetary motions	uniform,	uniform,	non-uniform,	non-uniform,
	circular	circular	elliptical	elliptical
Force on planets?	None,	None,	Yes,	Yes,
	natural	natural	magnetic	gravity
Kind of cosmos	geocentric,	heliocentric,	heliocentric,	centerless
	finite	finite	finite	infinite

5.2 Going beyond our own Galaxy

At the beginning of the 20th century, astronomers were aware of: planets, other stars, and fuzzy cloud-like patches of light called `nebulae'. Many of the nebulae had beautiful spiral shapes.

Despite Newton's concept of an infinite Universe, there was still a strong school of thought that our galaxy, the Milky Way, was all there was in the Universe.

The ``Shapley-Curtis'' debate:

Shapley - the spiral nebulae were distant clouds within the Milky Way.

Curtis - the spiral nebulae were other galaxies in their own right.

Curtis based his argument on the PRINCIPLE OF UNIFORMITY OF NATURE: all objects have similar properties until proven otherwise. Assuming this to be the case for the spiral nebulae, he showed that their range in angular size (apparent diameter on the sky) was so large that the smallest objects had to be at distances beyond our galaxy.

Resolution of the Shapley-Curtis debate: came in 1924 when EDWIN HUBBLE measured the distance to `Andromeda' - a spectacular spiral nebula in the northern sky - using variable stars within it. He derived a distance of 900,000 light years (=distance light travels in a year = 9,500,000,000,000kms), far beyond the most distant stars known in our galaxy.

EXTRAGALACTIC ASTRONOMY WAS BORN!!

Slipher: collected spectra of numerous spiral nebulae in the decade prior to 1924 $\rightarrow$ appeared to all be RECEEDING from us. When Hubble combined distance measurements he made to a number of spiral galaxies with Slipher's velocities, he found a correlation between the two: THE FURTHER AWAY A GALAXY WAS, THE FASTER IT WAS MOVING FROM US! This became known as Hubble's Law:

IMPLICATION OF HUBBLE'S LAW: WE LIVE IN AN EXPANDING UNIVERSE!
( $\Rightarrow$ a `Big Bang': expansion had to start at some point)

5.3 Advancement in technology/new technologies

5.3.1 Bigger telescopes

``Starlight is falling on every square mile of the earth's surface and the best we can do is gather up and concentrate the rays that strike an area 100inches in diameter'' (George Ellery Hale, 1928)

Lens-based (refractor) telescopes - limit to how big lenses could be made - heavy, difficult to support, optical distortions.

Mirror-based (reflector) telescopes - first used shiny metal surfaces as mirrors (speculum) - quickly oxidised (1800's). In the early 1900's, silver-on-glass mirrors were perfected and subsequently became the preferred technology for building larger & larger telescopes.

Date	1908	1917	1948	1993
Mirror size	60	100	200	400
(diameter in inches)

5.3.2 Detectors

These are the devices which capture and record astronomical light signals.

Early 1900's - photography developed and started to be used in astronomy, replacing the eyeball as a light detector. This had several advantages:

• provided a permanent record;
• could collect light over long periods (hours $\rightarrow$ nights) and so allow observations to go fainter;
• large format $\rightarrow$ map the whole sky.

1970's - electronic detectors begin to be used more and more in astronomy (silicon, TV, military technologies).

• up to 70-80 times more senstive than photography $\rightarrow$ fainter even still!
• limited by small format (cf. photography).

5.3.3 Computers

• telescope control (improve observing efficiency; allow cheaper construction methods to be used);
• data collection and processing (vast amounts of it!);
• astrophysical modelling (stars, galaxies, universes) via n-body simulations on very fast computers and supercomputures.

5.3.4 Space technology

(a)Launch telescopes into space

• open up regions of the electromagnetic spectrum not visible from the earth's surface (X-ray, ultra-violet, far infrared) $\rightarrow$ ``new windows on the Universe''.
• clearer vision (via higher resolution) in the optical (eg. Hubble Space Telescope).

(b)Send probes to other planets

• determine physical conditions - suitable to support life?
• understanding of how our solar system formed.