Transit of Venus

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, obscuring a small portion of the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. The duration of such transits is usually measured in hours (the transit of 2004 lasted six hours). A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is almost 4 times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth. Observations of transits of Venus helped scientists use the principle of parallax to calculate the distance between the Sun and the Earth.

Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that repeats every 243 years.
The first of a pair of transits of Venus in the beginning of the 21st century took place on 8 June 2004 and the next will be on 6 June 2012. After 2012, subsequent transits of Venus will be in December 2117 and December 2125.
Aside from its rarity, the original scientific interest in observing a transit of Venus was that it could be used to determine the size of the solar system by employing the parallax method.
The technique involved making precise observations of the slight difference in the time of either the start or the end of the transit from widely separated points on the Earth's surface. The distance between the points on the Earth was then used as a baseline to calculate the distance to Venus and the Sun via triangulation.
The transit pair of 1761 and 1769 were used to try to determine the precise value of the distance from the Earth to the Sun (astronomical unit (AU)) using parallax. In 1716 Halley suggested a high-precision measurement of the distance between the Earth and the Sun by timing the transit of Venus. Halley gained agreement for the project to go ahead, but died more than 25 years before the measurement took place. Numerous expeditions were made to various parts of the world in order to observe these transits; an early example of international scientific collaboration. In an attempt to observe the first transit of the pair, scientists and explorers from Britain, Austria and France travelled to destinations around the world, including Siberia, Norway, Newfoundland and Madagascar.^[18] Most managed to observe at least part of the transit, but excellent readings were made in particular by Jeremiah Dixon and Charles Mason at the Cape of Good Hope.
For the 1769 transit, scientists traveled to Hudson Bay, Baja California (then under Spanish control), and Norway. Observations were also made from Tahiti on the first voyage of Captain Cook, at a location still known as "Point Venus".
In 1771, using the combined 1761 and 1769 transit data, the French astronomer Jérôme Lalande calculated the astronomical unit to have a value of 153 million kilometers (±1 million km). The precision was less than hoped-for because of the black drop effect.

Estimating the Distance from Earth to the Moon

Using the knowledge of the length of the radius of the Earth and positioning two observers on the surface of theEarth, with one observer viewing the Moon directly overhead, and at the same time, the other observer viewing the Moon on the horizon. The distance between the two observers needs to be known as well. Then a calculation can be made of how far the Moon is away from the center of the Earth. [The calcualtion uses the pythagorus theorem which requires year 9 maths, and trigonometry from year 10 maths.]

Eratosthenes' Estimate of Earths's radius in 200BC

We owe a lot to the ancient Greek astronomers. Aristarchus (310-230 BC) argued for the Sun being the centre of the solar system. Hipparchos (190-120 BC) invented Trigonometry, and used the first trigonometric tables in his observation of the stars. He developed a star catalogue of over 850 stars.

Eratosthenes (276-195 BC) was a Greek astronomer, mathematician and poet. He travelled widely and knew that on the summer solstice at local noon in the Ancient Egyptian city of Swenet (known in Greek as Syene, and in the modern day as Aswan) on the Tropic of Cancer, the sun would appear at the zenith, directly overhead. He also knew, from measurement, that in his hometown of Alexandria, the angle of elevation of the Sun would be 1/50 of a full circle (7°12') south of the zenith at the same time. Assuming that Alexandria was due north of Syene he concluded that the distance from Alexandria to Syene must be 1/50 of the total circumference of the Earth. His estimated distance between the cities was 5000 stadia (about 500 geographical miles or 950 km). He rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. The exact size of the stadion he used is frequently argued. The common Attic stadium was about 185 m, which would imply a circumference of 46,620 km, i.e. 16.3% too large. However, if we assume that Eratosthenes used the "Egyptian stadium" of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 1%.

The Constellation of Orion

Orion, often referred to as "The Hunter," is a prominent group of stars – one of the largest constellations, most conspicuous, and most recognizable in the night sky.Its name refers to Orion a hunter in Greek mythology.

The map of the constellation is as seen from the northern hemisphere. This is upside down in the southern hemisphere.
It will be visible again in the night sky towards the east in spring.

The stars of Orion include Rigel, Betelgeuse, Bellatrix, Mintaka, Alnilam, Saiph and Alnitak.

Space Exploration

Space exploration is the use of astronomy and space technology to explore outer space.
Physical exploration of space is conducted both by human spaceflights and by robotic spacecraft.
It was the development of large liquid-fueled rocket engines during the early 20th century that allowed physical space exploration to become a reality. The rocket launch in the picture shows the take-off of Apollo 11, which took three astranauts (Armstrong, Collins and Aldrin) to the moon in July 1969. Click on the images including the detailed drawing of the many components which make up the giant rocket (101.6 meters tall), which contains the lunar module shown in the picture below.












Click on the image of Neil Armstrong on the surface of the moon.

The most distant galaxies

The most distant galaxies photographed with visible light were achieved with the Hubble orbiting telescope. This photograph is known as the "Hubble Ultra Deep Field". It was produced when the telescope was pointed towards a very small dark area of the sky, and the camera exposure was 1 million seconds. There are estimated to be 10,000 galaxies of all shapes in this image. The light from these galaxies has travelled 13 billion years to reach us. Click on the image for an amazing view into the past!

The Milky Way - Our Home Galaxy

The Milky Way galaxy is where our solar sytem is. It is believed to be a spiral galaxy similar to the one featured in the main picture heading this blogspot. It is believed to be 100,000 light years across, on average 1000 light years thick and estimated to contain more than 200 billion stars. If an imaginary model of the milky way were to be as big as the MCG (Melbourne Cricket Ground) then our solar system would be lmm across.


There are billions of galaxies in the Universe. The closest ones to the Milky Way include Andromeda and the Large and Small Magellanic Clouds. Andromeda is 2.5 million light years away. The picture to the right is the Andromeda galaxy.

The Solar System

The Solar System consists of the Sun and the planets and comets bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago.

The Sun makes up 99.86% of the total mass of the solar system.

The distances within the solar system are extremely large, so that it is very difficult to show a realistic diagram of the solar system.

One way of picturing it would be to build an imaginary model of the solar system as follows:[ the reason why it has to be imaginary will become clear quite soon!]
OK let's start: Imagine we are at the MCG (Melbourne Cricket Ground) and a small model of the sun is placed at the centre. Let's make the model of the sun 1cm in diameter (a bit smaller than the size of a 5 cent piece).

The Earth would then be about 1 meter away from the Sun and be very small, and the size of our model of our planet Earth would be smaller than the full stop at the end of this sentence.

Between the Earth and the Sun, would be Mercury about 40 cm from the Sun ( and virtually too small to see) and Venus would be about 60cm from the Sun (and about as small as the Earth).

Mars would be 1.6 meters out from the Sun.

Then there would be the asteroid belt, a sprinkling of dust forming a circle at about 3 meters from the Sun at the center.

Jupiter would be 6 meters away from the Sun and compared to the 1 cm diameter sun, Jupiter would be about 1mm in diameter (about the size of the head of a pin).

Saturn would be 10 meters from the Sun, and Uranus would be about 30 meters away from the Sun.

Neptune would be 45 meters from the Sun and Pluto would be 50 meters from the Sun.

So the limits of the edge of the solar system would be the boundary line. Given the smallness of the Sun and the planets in this model, most of the solar system is nearly empty space.

Now still using this model, how far out from the center of the MCG would the next nearest star be?

Using our imaginary model, the nearest star, Proxima Centauri (4.2 light years away) would have to be 265 km away from the center of the MCG. If we took it as being in a northery direction, it would be in NSW!

Alpha Centuri, would be 275 km from the Sun.

If we were to imagine using this model to also include where the centre of the milky way galaxy is. Then it would be 1.64 million km away from the center of the MCG!

Sirius

Sirius is the brightest star in the night sky. It appears bright for two reasons.

Firstly it is close to us compared to much brighter stars, which are much more distant. It is only 8.6 light years away [light travelling at 300,000,000 km per second, takes 8.6 years to reach us].

Secondly it is 25 times more luminous than the Sun (if Sirius was our star we would need to always wear dark sun glasses outside, as daylight would be 25 times as strong).

What the naked eye perceives as a single star is actually a binary star system, consisting of the very brightly luminous star Sirius A, and a faint white dwarf companion Sirius B.

A binary star system is one in which two stars are orbiting each other.

Sirius is in the constellation Canis Major (English translation = Big Dog).

It can be seen in Melbourne in late spring as a very bright star rising in the east early in the evening.

The nearest stars to the Sun

Proxima Centauri is the nearest star to the Sun. It is about 4.2 light years away [it takes light, which travels at 300,000,000 km every second, 4.2 years to reach us]. Proxima Centauri's mass is about an eighth of the Sun's. The red arrow shows its position in the constellation of Centaurus.



The white arrow below points to Alpha Centaurus one of the pointers to the Southern Cross.


Alpha Centauri is also close to the Sun: 4.37 light years away, and is the third brightest star in the night sky. It is slightly larger than the Sun.

Double-click on the images to enlarge them.

The Sun

The Sun is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun. The Sun mainly consists of Hydrogen (74% by mass) and Helium (24%). Its diameter is 109 times as big as planet Earth, and it is 150 million Km from Earth. The surface temperature of the Sun is over 5,500C while the temperature inside the Sun is over 15 million degrees. It is 26000 light years from the center of the Milky way and takes 250 million years to orbit the Milky Way.