Hubble Space Telescope:
-The Hubble Space Telescope was initially set for launch during 1986, but due to the redesign of the space shuttle following the Challenger accident, the launch was postponed.  During the delay-time, the telescope went through further tests and evaluations.
Appeared in Issue 21 dated 10/2001.

-The HST orbits the Earth at 600 km. It travels at the immense speed of 27 400 km/h.  In addition, the Earth orbits around the Sun at 107 000 km/h.  Despite these speeds, the telescope can still hold its target without varying more than 0.007 arc seconds for as long as 24 hours. This is achieved by using "guide stars information" and detailed observation instructions, sent to the telescope's precision guidance sensors via on-board computers.
- 10/2001, Issue 21

-If Hubble were to be trained onto the Earth with perfect focus, you would be able to distinguish two objects 12-15 cm apart. Thus it will be able to see a car's licence plate (but not necessarily the numbers on the plate). Because the telescope is too near to Earth, it will not be able to focus with such observations. The Earth's brightness can also cause damage to the telescope's instruments.
- 10/2001, Issue 21

-A few months after launch, it was found that the HST had a focusing problem. This defect was due to a 'spherical aberration' (found in incorrectly shaped mirrors). The mirror was too flat near the edge by about 1/50th the width of a human hair!   This resulted in light being spread over a too large area, instead of focusing on one sharp point. In 1993, the telescope underwent a shuttle servicing mission to repair the problem and corrective optics were installed.
- 10/2001, Issue 21

- Some of the objects visible in Hubble Space Telescope images are nearly four billion times fainter than the limits of human vision.
04/2001, Issue 9

International Space Station:
-The ISS will be the largest manned object ever to orbit the Earth. When completed, the pressurized living and working space will be just more than the volume of two Boeing 747's. It will then weigh an incredible 453 tons and will be as large as a rugby-field.  
08/2001, Issue 19

-The weight of the electric hardware system will be 70 tons.  Four U.S. photovoltaic modules supply the station with 23 kW of power each.
08/2001, Issue 19

-A great risk associated with low-gravity is bone and muscle loss.  The ISS crew must undergo strenuous exercise to decrease this risk and stay fit.
08/2001, Issue 19

-The huge robotic crane (Canadarm 2) which was contributed by Canada, is 18 meters (58 feet) long and has the capability of lifting up to 125 tons of equipment.
08/2001, Issue 19

-The station's electric system will be connected by more than 12 km of wiring. Fifty-two computers will control the systems on the station. There will be more than 400 000 lines of software for 16 of those computers alone. Two computers are dedicated to keeping the station in a proper orientation while it orbits the Earth.
08/2001, Issue 19

Space Shuttle:
-The outside skin of the shuttle can reach temperatures of up to 1 650 °C upon re-entry into Earth's atmosphere.
When the liquid hydrogen in the Space Shuttle's main engine is burned with liquid oxygen, the temperature in the engine's combustion chamber increase to more than 3 300 °C. Compare these figures with the Sun's surface temperature of  6 000 °C.
07/2001, Issue 18

-After lift-off, it takes only about eight minutes for the Space Shuttle to accelerate to its orbital speed of more than 27,358 kilometers per hour.
07/2001, Issue 18

-The turbopump on the main engine of the Shuttle is so powerful that it could drain an average family-sized swimming pool in only 25 seconds.
07/2001, Issue 18

-The main engine of the Space Shuttle weighs only 1/7th as much as a train engine does, but delivers as much horsepower as 39 locomotives.
07/2001, Issue 18

-Kennedy Space Centre's two Crawlers are the largest tracked vehicles on Earth. They are used to transport the shuttle on its platform from the Vehicle Assembly Building (VAB) to the launch pad.
07/2001, Issue 18

-Each Crawler weighs an incredible 2 721 metric tons!  But, they travel at only 1.6 km/h when loaded. It takes the vehicle almost five hours to travel from the VAB to the launch pad with the shuttle-system on top.  (The two shuttle launch pads are respectively 5.5 and 6.8 km away from the VAB)

The Crawler, carrying the Mobile Launch Platform
with the Shuttle-rocket system upon it.
Image Credit: NASA
07/2001, Issue 18

Manned Spaceflight:
-During a mission in space, astronauts can temporarily become 5 cm taller. The reason for this: without all the pressure of gravity, the cartilage discs in the spine expand like sponges - increasing the body's total length. When we sleep, the same happens, only not on such an immense scale.
05/2001, Issue 13

-In space around the Earth, objects in direct sunlight can heat up to temperatures of about 121 °C. When they are shielded from the Sun however, they can cool to around -156 °C. Astronauts in the space shuttle or in their space suits, work in normal temperatures of 21-27 °C because of the thermal control systems in the shuttle and their suits.
05/2001, Issue 13


Rockets:
-It is better for rockets to launch from positions near the equator: they can take full advantage of the Earth's spin - allowing them to carry heavier payloads than those taking off from other latitudes.

Other:

-During 1968 and 1972, NASA launched the first two successful NASA satellites designed for observing the stars. These Orbiting Astronomical Observatories provided an immense knowledge-base for future spacecraft with similar duties.
- 10/2001, Issue 21

The Sun:
-It takes light waves from the core of the Sun on average about one million years before they reach Earth. The light wave starts out as a gamma ray photon, moving at the speed of light. But, the gamma ray photon undergoes so many collisions with sub-atomic particles on the way, that by the time it escapes the solar furnace, it has already lost so much of its energy, that it emerges as a photon of ordinary visible light.
06/2001, Issue 16

-The diameter of the Sun is about 1,4 million kilometers. This is 109 times more the Earth's. (The Earth's diameter is 12 756 km) The Sun's volume is big enough to hold more than 1.3 million Earths.
06/2001, Issue 16

-The Sun contains more than 99.8% of the total mass of the whole solar system (with Jupiter taking up most of the rest).
06/2001, Issue 16

-The Sun is about 4.6 billion years old. It has already used up almost half of its hydrogen supply in its core. The Sun will run out of this hydrogen supply in about 5 billion years. It will then expand and engulf all of the inner planets. After its destruction, a planetary nebula will most probably be left over, with the Sun as a white dwarf in the centre.
06/2001, Issue 16

-The Sun's rotation period at its equator is only 25 days.
05/2001, Issue 10

-The temperature at the centre of the Sun is about 15 000 000° C. A physicist once calculated that if a pinhead was made to be the same temperature as the material at the core of the Sun, it would set everything alight for 100 km around it.
05/2001, Issue 11


Venus:
- Venus rotates very slowly. It takes the planet approximately 243 days to rotate once. (where it takes the Earth only 24 hours)
04/2001, Issue 9

- In ancient times, Venus was called "Hesperus" when it was a morning star and "Phosphorus" when visible during sunset as the evening star. The planet itself was named after the Roman goddess of beauty and love. As apparent from the paragraph about Venus above, the named might have been a bit misplaced. Venus is known to be a most inhospitable place! It looks very beautiful from Earth during sunsets though.

- Many surface features of Venus became visible after Earth's spacecraft visited the planet with radar technology. To identify these features (e.g. mountains, volcanoes, canyons, craters etc.), it was decided by the International Astronomical Union that they should all be named after accomplished women from all of Earth's cultures. An exception is the Maxwell Mountains (Maxwell Montes - 11.3 km high), a huge mountain, named after the physicist James Clerk Maxwell. He and only a few others are known as the only men on Venus! (Spies perhaps?)

- Venus spins on its axis in the opposite direction as Earth and most other planets. This means that the Sun rises in the west and sets in the east on Venus.

- The atmosphere of Venus is enormously thick. This thickness gives rise to a very high greenhouse effect. This means that light waves (heat) from the Sun comes into the atmosphere, but get trapped inside, unable to escape. This is almost the same as a car parked in the Sun: it gets quite warm inside, the heat unable to get out again. The planet is trapped in an endless heat cycle - diminishing hopes that we can establish a research base on its surface.
- 04/2002 , Issue 23

Earth:
-Not only are the oceans affected by the gravitational pull of the Moon. Twice daily, the continents can lift up to 15 cm when the Moon is directly overhead.
- 09/2001, Issue 20

-Each year, North America and Europe move 4 cm away from each other due to the movement of continent-bearing plates in the Atlantic. 
- 09/2001, Issue 20

-While observing a lunar eclipse, one can clearly see the curved shape of the Earth's shadow, which is irrefutable, solid-state evidence that the Earth is indeed round!
- 09/2001, Issue 20

-The lowest spot on the Earth's surface is called the Marianas Trench, a deep-ocean trench situated east of the Mariana Islands in the western North Pacific Ocean.  It is 10 914 meters below sea-level.  Of the Earth's 20 major trenches, 17 are located in the Pacific.
- 09/2001, Issue 20

-The magnetic field of the Earth sometimes flips - changing polarity so that north becomes south.  The frequency of change ranges from intervals between 5 000 to even 5 million years.  The exact cause why this happens is still a mystery.  Historic data points we are overdue for a flip.. The Sun on the other hand, changes its magnetic field once every 11 years. This is consistent with the sunspot cycle and occurs during the solar maximum.
- 09/2001, Issue 20

-The Earth is rotating slower. Because of the Moon orbiting the Earth, the oceans undergo friction with the land (tidal forces), which brakes the rotation speed of the Earth and increases our days with 0.0023 seconds per century. 900 million years ago, one day on Earth were only 18 hours long!
05/2001, Issue 13

-Along the equator on Earth, dawn arrives at a speed of 1 600 km/h - the rotation speed of our planet. (A large passenger plane travels at 950 km/h) On the Moon however, dawn arrives at only 16 km/h - enough for a man on a bicycle to keep up with it.
05/2001, Issue 11

-Of all the planets in our solar system, Earth has the highest average density (5.52 g/cubic cm), Saturn has the lowest (0.69 g/cubic cm). (The density of liquid water is 1 g/cubic cm.)
03/2001, Issue 2

- The earth's average velocity orbiting the sun is 107 220 km per hour.
02/2001, Issue 1

The Moon:
-The far side of our Moon was not seen at all until 1959 when the Soviet spacecraft, Luna 3 flew by and took pictures.
06/2001, Issue 15

-Only one side of the Moon faces towards the Earth at all times (called synchronous rotation). The reason? The Moon spins around its own axis at a very slow rate. It takes the Moon 29.5 days to spin around its own axis (the Earth takes only 24 hours) The Moon also takes about 29.5 days to make one orbit around Earth. For example: Person M equals the Moon and person E equals the Earth. If M walks around E once, and faces him the whole time, M would have spun himself around once!
06/2001, Issue 15

-The footstep-marks of the Apollo astronauts who walked on the Moon, might stay unscathed for a very long time. Because the Moon has no atmosphere, there cannot exist any atmospheric pressure, which means there cannot be any wind. There are no tectonic or volcanic activity either. Now, only meteorite impacts heaving up dust can disturb this human-monuments.
06/2001, Issue 15

-The last human mission to the Moon occurred on December 12, 1972. They also used a lunar rover while collecting rock samples and performing experiments.
04/2001, Issue 8

-The Moon is moving away from the Earth at about 3-4 cm per year. (The moon is roughly 380 000 kilometers away from Earth.)
05/2001, Issue 10


Mars:
-Mars has 2 moons, Phobos and Deimos. The second moon, Deimos, is an irregular shaped piece of rock 15 km across. Because of its small size, its gravitational pull is so weak, that an astronaut would be able to overcome the escape velocity by running at 11 km/h and simply JUMP of the moon!
04/2001, Issue 6

-Mars has the largest mountain in the Solar System named Olympus Mons. It is 26 km high, almost 3 times higher than Mt. Everest.
03/2001, Issue 2


Jupiter:
-Jupiter has no seasons. Planets can only have seasons if they are tilted on their axes. (Earth is tilted 23,5°) Jupiter's axis of rotation is only tilted 3°.
05/2001, Issue 14

-The most volcanically active body in the solar system besides the Earth, is Jupiter's moon Io. Active volcanoes were first discovered by the Voyager satellites.
04/2001, Issue 8

- It takes the gas giant Jupiter only about 10 hours to complete one rotation. This means that a day on Jupiter is only 10 hours long, compared to the Earth's 24 hours. Jupiter has the shortest day of all the planets, although it is the largest planet...it will take 1 321 Earths to fill up one Jupiter!
03/2001, Issue 4

- The largest moon in the solar system is Ganymede, one of Jupiter's moons. Ganymede is even larger than the planets Mercury and Pluto.
03/2001, Issue 3

The moons of Jupiter: (05/2002 , Issue 24)

Jupiter has many moons and more could be discovered as time progress. Usually these moons are nothing more than small irregular pieces of rock caught up in the planet's gravity. All the moons are composed of rocky substances (not gas like Jupiter). Jupiter has some of the most intriguing moons in the solar system. Some are so mysterious, that dedicated future spacecraft missions to them might be planned.  Its 4 biggest moons are described here:

Ganymede:  This is the biggest moon in the solar system. (Diameter = 5 262 km, Earth's moon = 3 476 km).  If Ganymede orbited the Sun and not Jupiter, it would have qualified for a planet! (Pluto's diameter is 2 300 km).  Complex geological processes formed Ganymede's mountains, craters, valleys and it still bears lava flow.

Callisto:  Second largest moon of Jupiter, and third largest in the solar system.  It is immensely cratered and has little mountains. It has no atmosphere and its icy surface suggest an ice crust about 200 km thick.

Io: This moon is indeed a bizarre world.  The Voyager spacecraft observed, for the first time on any other planet, erupting volcanoes!  Plumes can be seen shooting into the air 300 km above the surface. This is due to Io being subjected to huge tidal forces, by its companion moons and by its great keeper, Jupiter. It is 422 000 km from the giant.

Europa:  Like Mars, Europa is a prime target for possible forms of microbial life or at least fossilized traces of life and has intrigued scientists for a long time.  It has a young and smooth crust (upper surface) composed of icy substances.  Parts of the crust may be liquid and their might even by a vast sub-surface ocean underneath. Future robotic missions, in the form of submersible vehicles, may penetrate this crust one day and travel through the "waters". We can only speculate what it might find.

Saturn:

- Winds in the upper atmosphere of Saturn can reach speeds of up to 1 800 km/h. These winds together with rising heat waves from the planet's interior, causes the streaking lines visible across the planet.
- 06/2002, Issue 25

- Saturn's density is less than that of water. The theoretical implication of this is that if you could find a big enough ocean and place Saturn on it, the planet would float!
- 06/2002, Issue 25

- Because Saturn is composed of gas, you will start falling through its "surface" if you attempt standing on it. As you go deeper into its atmosphere, the pressure will increase until it is totally unbearable and you'll get crushed. Going still deeper, at some time the gas might start to feel like a liquid. It is believed that the core of Saturn consists of metallic-like melted hydrogen, which gives rise to extremely high temperatures. A hot rocky core is probably at the very centre.
- 06/2002, Issue 25

- Cassini will map Titan's surface using radar. There is probably a lot of chemistry going on in Titan's atmosphere - a mirror image of how it might have looked on Earth during its formation process. Titan might even be a prime candidate for primitive organic life - though the place is extremely cold - it is well know that certain life forms here on Earth manage to live in the harshest of environments with neither adequate sunlight nor oxygen to sustain them. Future missions may reveal some interesting finds…
- 06/2002, Issue 25

Uranus:

-Why does Uranus and Neptune have such a characteristic blue-green appearance? This is because of the methane gas in their atmospheres.  The methane gas particles absorb the more reddish light and reflect the blue-green colours more strongly. Light waves being absorbed won't be visible to us - but those reflected will.

- The gas planet Uranus is tilted on its axis more than any other planet. It lies on its side, tilted at an angle of 98° - the Earth is tilted only about 24°.  This huge tilt may be due to a collision with a large solar system body (a planet or moon) very early in its history. Seasons on Uranus last almost 20 years at a time.
08/2002 , Issue 26

Neptune:

-Though it may not rain cats on Neptune, it might rain diamonds.  This is according to experiments performed by a University in the USA.  Because of the huge pressures inside the core of the planet, the methane molecules in the atmosphere starts to decompose (break apart). This means that methane's four hydrogen atoms come apart which leaves the carbon atom behind to bind with other carbon atoms and under extreme pressure they may form very small diamonds, falling deep into the planet.  Who is up for starting the first mine.?

-Six of Neptune's 8 moons were discovered by Voyager 2. Triton is the largest and orbits Neptune in a direction opposite to the planet's own rotational spin. Triton is slowly but surely getting nearer and nearer to Neptune - it is said that it will collide with the planet in about 10 to 100 million years. This collision will result in Neptune also getting huge magnificent rings like Saturn - it may even look more spectacular.

-Like the other gas giants, Neptune also has extremely stormy weather in its atmosphere. The fastest winds in the solar system can be found on Neptune - reaching speeds of up to 2 000 km/h.
08/2002 , Issue 26

Pluto:
-Pluto is now known as a "dwarf planet" and not an ordinary planet anymore (since August 2006).

-The Sun looks 1 600 times fainter from Pluto than it does from the Earth. Pluto weighs only 0.22% the mass of the Earth - it will take almost 454 Plutos to equal the mass of the Earth.
05/2001, Issue 14

If you visit Pluto remember some warm clothes! The planet has temperatures ranging between - 230° C and - 200° C. (yes, all in the minus).

Pluto was named after the Greek god of the underworld, Hades. The Disney figure - the dog Pluto - was named after the planet a few years after its discovery.

Pluto is even smaller than Earth's moon; Pluto's diameter is 2 300 km and our Moon = 3 475 km. Pluto is the only planet in our solar system which has not been visited by a space probe.
10/2002, Issue 27

- In the twentieth century, two objects have hit the Earth's surface with enough force to destroy a medium sized city. By pure luck both have landed in sparsely populated Siberia.
02/2001, Issue 1

-In 1960, an American rocket which went off course after launch, crashed in Cuba and killed a cow. The Cubans gave the cow
a very official funeral in respects of this victim of 'imperialist aggression'!
05/2001, Issue 12

-In 1968, a small piece of a re-entering Russian rocket fell through the window of a house in southern England. In 1979, a piece
of a disintegrating satellite made an extra hole in a golf course at the seaside resort of Eastbourne in Britain!
05/2001, Issue 12

-The only person known to have been hit by a meteorite is Mrs. Hewlett Hodges of Alabama, U.S. In 1954, a 4kg meteorite rock crashed through her roof, bounced off a radio and struck her on the hip, causing serious bruises but no permanent injury.
05/2001, Issue 12

Meteors

●Meteors have a more "popular" name: shooting stars.  It is not shooting stars at all though, but merely small fragments of sand and dust entering Earth's atmosphere.
In the basic sense of the word, when a meteor enters the atmosphere it starts to heat up due to friction with the gas molecules. The friction becomes so enormous that a huge amount of energy is released - the poor piece of dust burns up in the atmosphere!  On Earth, we see this burn-up process in the form of shooting stars or more appropriate, meteors. 

●Meteors entering the atmosphere can start to travel at an average speed of 70 km per second. This is equal to traveling once around the Earth in almost 4 minutes!

●Sometimes it happens that an object bigger than dust enters the atmosphere. This might be a space rock or even a part of a disused artificial satellite. Then you will probably be treated to a spectacular fireball. If the object is large enough, it might even hit the ground. When a meteor hits the ground, the fallen object is called a meteorite.  The "shooting stars" we see during a meteor shower are all meteors - fine pieces of dust.
-Issue # 28, December 2002

- Every second on our own star the Sun, about 700 million tons of hydrogen atoms get converted into 695 million tons of helium atoms!

- More than 50% of all stars are in fact, double or multiple stars. Our Sun is a single star - it does not have an orbiting companion. A double star is two stars that orbit a central point of gravity. The star Castor in Gemini is in fact, six stars orbiting one central point. Anyone who has read Isaac Asimov's "Nightfall" will know that a solar system like this can have interesting implications: if there is a planet in that solar system, it might never see night - or at least very rarely. Just imagine how the sky would have looked if in our own solar system, Jupiter was a star!

Types of Stars
- You get different types of stars with different characteristics. Henry Russel and Ejnar Hertsprung devised a diagram on which all these types of stars were plotted. (The Hertsprung-Russel Diagram).
Red Dwarfs: These very common stars are at the faint, small and "cool" end of the categories and are difficult to observe. They live very long - tens of billions of years.
White Dwarfs: These stars are even smaller than red dwarfs. They can even be as small as Earth, but can weigh as much as our Sun. They are old stars which have used up most of their nuclear (hydrogen) fuel.
Main Sequence Stars: Most stars, including our Sun, fall into this category. These are "middle-of-the-road" stars, but some can be 20 times larger than our Sun.
Red Giants: These are also rather common stars, but not as many as main sequence stars. Though they have the temperature of a red dwarf, they are huge compared to stars like our Sun. They don't usually live very long - maybe only a few hundreds of millions of year - because they burn up their nuclear fuel so quickly.
Supergiants: These are rare but incredibly large stars. The heaviest of these are the ones that can explode as supernovas and form neutron stars or even black holes...
Our Sun do not weigh enough to become a neutron star or black hole. You need at least a star with 1.4 the mass of the Sun to form a neutron star. If a star is too dense to become a white dwarf or a neutron star, it can collapse into a black hole...

05/2003, Issue 30

Galaxies:
-Sometimes it happens that galaxies pass very near to each other when they move through space. When these galaxies get caught up in each other’s gravitational stranglehold, it can result in the two ‘colliding’. This may take millions or even billions of years. The distance between stars in galaxies is however, immense! If you put three grains of sand on a rugbyfield, the field will be more closely packed with sand than our galaxy is packed with stars. This means that if galaxies collide, the chance of their stars colliding is almost zero! Our galaxy might roughly contains 200-400 billion stars.
06/2001, Issue 17

-Our galaxy is part of a local group of galaxies, consisting out of 30 galaxies (called the "Local Group"). The Local Group is part of a still larger supercluster of galaxies, whose other members can be found in the direction of the constellations of Coma Berenices and Virgo. We have seen an incredible number of other superclusters of galaxies scattered throughout the Universe. There are tens or maybe even hundreds of billions of galaxies in the universe. They tend to form these superclusters, held together by gravitational forces. Can we possibly even think of how immense the universe really is? More and more sophisticated satellites are being launched into orbit to try and provide us with some answers on the many questions we still have.
06/2001, Issue 17

Neutron Stars:
-Neutron stars exist purely out of highly compressed and dense neutron atoms. These stars are incredibly heavy: one milli-liter of the star's matter will weigh 238 times the mass of all the people on Earth together. Neutron stars are also very small: usually no more than the size of a city.
05/2001, Issue 14

Nebulae
- Although the space between stars in the Milky Way is extremely large and empty, it contains a diffuse medium of gas and dust called "interstellar medium (ISM)", (nebulae for instance). These gas clouds may look very large, but can contain on average only 3 atoms per square meter! In one of these clouds in the Sagittarius constellation, astronomers have found ethyl alcohol molecules - enough to fill 10 000 million million million million bottles of whisky!
04/2001, Issue 7

- The stars of the North

- There are altogether 88 official constellations. We can see most of them either fully or partially from South Africa.  But 6 of these constellations are true northern hemispheric constellations. They cannot be seen in their completeness by observers in SA. Be sure to observe them when you visit the countries of the northern hemisphere.  Their names are:
Camelopardalis  (The Giraffe) - Named the 'camel-leopard' by the Greeks.
Cassiopeia  (The Queen) - In mythology, queen of ancient Æthiopia, wife of Cepheus and mother of Andromeda.
Cepheus  (The King) - The king of Æthiopia, husband of Cassiopeia, father of Andromeda.
Draco  (The Dragon) - Guardian of the entrance to the golden apples, slain by the hero Hercules.
Ursa Major  (The Great Bear) - One of the oldest constellations, numerous legends associated with it.
Ursa Minor  (The Little Bear) - Named Arcas in legends, following its mother through the sky.

- You might have heard about a star pattern called "the Big Dipper". Where is this pattern exactly?  It consists of a group of 7 stars situated in the constellation of Ursa Major.  It is also known as the "Plough".  One Native American legend sees the Big Dipper as seven hunters pursuing a bear.  The early Britons imagined the Big Dipper as the chariot of King Arthur.  Many other legends from many cultures are told about this specific star pattern.

- Another well-known object in the northern hemisphere is the star called "Polaris", (alpha Ursa Minoris).  This star is also known as the northern pole star. It can be found in Ursa Minor. Polaris is situated only 1 degree from the real northern celestial pole. (If you draw an imaginary line from the middle of the North Pole into space, it will point to the north celestial pole).  All the other stars in the northern hemisphere seem to revolve around the north celestial pole, as the Earth turns on its axis. In the south, we do not have such a relatively bright star to show us where the south celestial pole lies. But the nearest star to that position was named "Sigma Octantis", a faint star of magnitude 5.4, visible through binoculars.

Star Names

The proper names of the major and brightest stars come mainly from the ancient Arabs and Greeks before 0 B.C.  We still use them to this day.  In modern times, stars are also designated in number-form by the IAU - the International Astronomical Union. (No one else may name stars - it will be considered void!)  Not all stars have popular names, but at least all those visible have designations.
The star Sirius in the constellation of Canis Major (the big dog) comes from the ancient Greeks and means "scorching" - probably because it is the brightest star in the sky.  Stars also have Greek alphabet letters assigned to them. These letters were assigned by Johann Bayer at the start of the 17^th century.  The brightest star of a constellation is usually designated Alpha <constellation name>, the second brightest Beta <constellation name> etc. Sirius is also called Alpha Canis Majoris.

List of the 15 brightest stars visible from Earth and their magnitudes. The less the magnitude, the brighter the star:

 04/2003, Issue 29

The Ocean Tides

The Moon has a very important effect on the oceans and even the weather on Earth. But first, let's look at some distances:
The Earth's diameter is 12 756 km. The deepest part of the ocean, the Marianas Trench, goes down 11.7 km under the sea's surface.  Mount Everest, the highest peak on Earth, is 8 km in height.  Though the atmosphere stretches upwards a few hundred kilometres, 99% of the atmosphere's weight is concentrated within 30 km above sea-level. If the Earth was an apple cut in half, then its skin would represent the atmosphere. The International Space Station orbits in space, at a distance of 400 km above the Earth. The Moon is 380 000 km away from Earth.

The Moon and Sun's gravitational pull on the Earth causes the oceans to shift their weight.  The Moon will pull the water towards it as illustrated by the following simplified, but exaggerated diagram. A cross-cut of the Earth is shown:

At any given place on Earth, the tides occur almost one hour later each day. The Moon also appears to move one hour each day overhead. Also remember the rotation of the Earth once every 24 hours around its axis.

The Moon does not only pull the oceans towards it, it exerts this pulling force on every object on Earth. Even the continents have "tides" - they can shift upwards about 20 cm.
Spring tides (Springgety in Afrikaans) occurs when the Earth, Moon and Sun are in a straight line (during new and full moon). The Sun exerts a pulling force together with the Moon to create high high tides and low low tides (diagram below).  Neap tides occur during first and last quarter: a "less than average" tide.