- The Astronomy across time.
- Dimensions and distances in Astronomy.
- Galaxies and clusters. The Milky Way and the Local Group.
- The Stars and the constellations.
- The Solar System: the Sun, the planets, the dwarf planets and the SSSBs.
- The movements of the Earth and their effects.
- The Moon and its phases.
- The eclipses.
- The Ecosphere.
|Gravity||Every object in the Universe exerts a force of attraction upon the objects around it. This force is known as gravity. The Earth is retained by the Sun because of the Sun's gravity; the Moon is retained by the Earth because of the Earth's gravity, etc.|
|Billion||In most English-speaking countries a billion equals one thousand million; you can write it as 109. Likewise, a trillion equals one million millions, and you can write it as 1012.|
|Light year||Distances in the Universe are huge, and so they are measured in huge units. A light year is a distance unit that equals about 9.5 trillion km. It is the distance covered by the light in one year.|
|Astronomical unit||It is another distance unit. It is the distance between the Sun and the Earth, and equals some 150 million km.|
Galaxies and Clusters
The Universe is about 14 billion years old, and is formed by more than 100 billion galaxies. A galaxy is a huge system of stars, interstellar gas and dust. Typical galaxies contain from ten million to one trillion stars, all orbiting a common centre of gravity. Some galaxies are elliptical shape, some are spiral, others are irregular. Galaxies are usually separated from others by distances on the order of millions of light years. Groups of galaxies gravitationally attracted between themselves are called galactic clusters.
The Solar System is located in the Milky Way galaxy, a spiral galaxy with a diameter estimated at about 100,000 light years, containing approximately 200 billion stars. The Milky Way belongs in a cluster of over 30 galaxies known as the Local Group. The galaxy of Andromeda is the nearest to the Milky Way and the biggest one in the Local Group.
The Solar System resides in one of the Milky Way's spiral arms, known as the Orion Arm, at about 27,000 light years from the galactic centre. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years.
Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are alone in the sky (the Sun, the North Star), others have companions (Sirius, which is a binary star system, and Alpha Centauri, which is a ternary star system). The nearest star to the Sun is Alpha Centauri C or Proxima Centauri. The brightest star in the Northern hemisphere's night sky is Sirius.
Not all stars are the same: stars come in all sizes, brightnesses, temperatures and colours:
- The colour of a star is due to its temperature. A blue or white star is hotter than a yellow star, which is hotter than a red star.
- The brightness of a star depends on two factors: (a) its distance from us and (b) its luminosity: how much energy it puts out in a given time, which, in turn, depends on its size. Think on a street lamp, which puts out more light than a hand torch. That is, the street lamp is more luminous. But if that street lamp is 1 km away from you, it will not be as bright, because light intensity decreases with distance. In this case, a hand torch held 10 m away from you would be brighter than the street lamp. The same is true for stars.
Constellations are groups of stars visibly related to each other in a particular pattern. Some well known constellations contain familiar patterns of bright stars. Examples are Ursa Major (containing the Big Dipper or Plough), Orion (that resembles the figure of a hunter) and Cassiopeia (with the shape of a "W"). The stars of a constellation, although appearing to be very near, may be millions LY away one to another.
The Solar System
The Solar System is the stellar system formed by the Sun and the group of celestial objects gravitationally bound to it:
- the eight planets and their 162 known moons,
- five dwarf planets and their six known moons, and
- thousands of small solar system bodies (SSSB).
The Sun is the main component of the Solar System, a star that contains 99.9% of the Solar System's mass. The Sun releases enormous amounts of energy in the form of electromagnetic radiation, which includes visible radiation (light), ultraviolet radiation and infrared radiation.
The planets are the biggest objects orbiting the Sun. Their orbit is almost circular. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky planets; the four outer planets are gaseous giant planets with a small rocky core. All planets but the two first are orbited by natural satellites (usually called "moons"). The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology. The following table shows some major magnitudes measured relative to the Earth:
|Diameter (relative to the Earth)||Mass (relative to the Earth)||Distance to the Sun (in AU)||Revolution period (in Earth's years)||Rotation period (in Earth's days)||Moons|
The dwarf planets are also rocky objects orbiting the Sun, smaller than the planets, but bigger than asteroids. There are currently (20-sep-2008) five dwarf planets in the Solar System; the two best known of which are:
- Ceres, formerly considered the Solar System's biggest asteroid;
- Pluto, a trans-neptunian object formerly considered the Solar System's ninth planet;
The SSSBs comprise several types of celestial bodies, the best known of which are:
- Asteroids = planetoids = minor planets. They're the smallest rocky bodies orbiting the Sun. Unlike planets and dwarf planets, they are not spherical, but irregularly shaped. Most of them occupy orbits between the ones of Mars and Jupiter, and make up the asteroid belt. The biggest asteroid is called Vesta.
- Comets, very small icy trans-neptunian objects that orbit the Sun in very eccentric orbits. When a comet approaches the Sun, its icy surface begins to boil away, creating two long tails, one of gas and another of dust, which are often visible with the naked eye. Two well known comets are Halley and Hale-Bopp.
Presentation: Why the Moon?Source
Definitions, images and pronunciation for almost everything in celestial bodies, astronomical observations and astronautics.
The Universe, the galaxies, the stars, the Solar System, and more.
20 things you probably didn't know about living in space.
Exploring the Universe
An historic timeline of Mankind's awareness and discovery of other planets. A most excellent resource by NASA.
2009 was the 40th anniversary of the first lunar landing. This superb website was set up to commemorate that historic event. The mission is recreated in precise detail right from pre-launch with actual voiceovers. You can change the views and also click on hotspots to view related media galleries. The site is divided into 11 sections where you can watch photos, videos, and other archival content.
Google Earth Tour that describes the landing, moon walks, and the return to space of Apollo 11 with excellent visual aids and great narrations.
Don't forget to check the links to the Apollo 11 and Apollo 13 missions.
A look back at the top 10 American space missions that didn't make it.
A great collection of 32 high quality photos of the IIS, compiled for its tenth birthday.
Nine maps, the first one showing the nearest stars. The others slowly expand out until we have reached the scale of the entire visible Universe.
A classic movie that shows the amazingly huge size of the Universe parting from a small square one metre long.
A collection of 25 high quality photos of the Universe from the amazing Hubble Space Telescope.
25 commented photographs, some old, some new, to celebrate the traditional western Christmas Season.
Third annual Hubble Space Telescope imagery Advent Calendar, with 25 new stunning photos.
Some of the most important and famous images taken by the Hubble explained in detail. Visiting this site is a must.
The entire collection of the images taken by the Hubble classified in categories.
Ongoing project that features a high resolution sky map with celestial objects that are outside our solar system mapped out.
Stars, Galaxies, Nebulas and Constellations
Visualize an animation that shows how the night sky changes along the year at Exploring Earth online book.
Simulated planetarium that allows you to observe stars, constellations and planets from any specific location.
360-degree panoramic image, covering the whole of the vault of heaven.
Animation showing the processes that take place at the end of the life of a high mass star.
The red supergiant star Betelgeuse is getting ready to go supernova, and when it does Earth will have a front-row seat.
The Solar System
Fly around the Solar System's planets and explore the constellations in 3D and real time with this excellent interactive model.
Visualize an animation showing the origin of the Solar System from a nebula at Exploring Earth online book.
Wikipedia's Wikijunior online book on the Solar System. Comprehensive, aimed to kids, but still unfinished.
Wikipedia's article on the Solar System. Very up to date.
Full information on the Solar System offered by the scientists from the NASA.
Collected here are 32 images gathered from around our solar system, at scales ranging from mere centimeters to millions of kilometers.
Images of the Solar System bodies taken by the NASA missions.
Some of the most intriguing facts in our stellar system.
Sun's structure, energy and influence on the Earth.
Beautiful photo taken by astrophotographer Alan Friedman that shows sunspots, giant convection cells and prominences silhouetted against the sky.
A collection of some of the most amazing photos ever taken of our star.
Amazing animation showing the movements of the planets from different perspectives.
Sensational images of Saturn showing the ringed planet in incredible detail.
During the equinox, the sunlight casts long shadows across Saturn's rings, highlighting previously known phenomena and revealing a few never-before seen images.
A wonderful selection of some of the best martian images ever made.
Stunning satellite images of the Red Planet reveal avalanches, sand dunes and more.
Landing on Mars is an inevitability. This website simulates a tour through the Martian landscape and a human colonized habitat. The walkthrough is a lesson in the science, technology, and design challenges that will be required to land man on the Red Planet.
Visualize an animation that demonstrates that the Earth rotates around itself at Exploring Earth online book.
Visualize an animation that shows how the Earth rotates around itself at Exploring Earth online book.
A great series of activities to learn why and where it is daytime or night-time in any given moment.
Want to know how the Earth looks like right now?
Visualize an animation showing how the Earth revolves around the Sun at Exploring Earth online book.
Learn how the relative position of the Earth to the Sun conditions the climate and the daylength.
Observe the Earth's maps of temperatures, volcanoes, earthquakes, water vapor and more, at Exploring Earth online book.
Visualize the Earth system as a set of four overlapping and interacting spheres, at Exploring Earth online book.
A great series of activities to study how the four spheres of the Earth interact, at Exploring Earth online book.
Fantastic article that unravels some of the most significant facts about our planet... or did you already know that the Everest is not the biggest mountain?
The geoid is the Earth depicted according to the gravty of its surface. ESA's GOCE satellite shows it with an unprecedented accuracy.
Those were the great news more than 2,200 years ago. They took a time to spread around the globe, though.
If you are interested in the Moon or the Apollo landings, Google currently has a wonderful educational tool regarding the Moon that will interest everyone.
Videos, images and lots of info to learn almost everything about our only natural satellite.
The phases of the Moon explained through diagrams.
Images of the Moon taken by the NASA missions.
Taken by the Apollo 12 crew, in the second manned mission to the Moon ever.
Taken by the Apollo 17 crew just a few days before the man left the Moon for the last time.
Exploring the Universe
Listen to one of the most frightening moments ever lived in space.
Reading: Astronomers Await a NovaSource: Space.com
The Star of Bethlehem
At this time of year it seems almost traditional for stargazers to ponder the age-old question of the origin of the Star of Bethlehem. The Star's appearance some 2,000 years ago is quite possibly one of the best-known celestial events in all of recorded history.
The topic has universal fascination, and is why Christmas Star shows still play to packed planetarium houses.
Perhaps the simplest answer that can be offered is that the Star might have been a nova: a new star suddenly blazing forth where no star had previously been seen. While for the most part such objects are really dying stars having a final fling of glory before descending the long road to ultimate extinction, there are some stars that go through such contortions more than once.
One such star is long overdue to pop and could do so at anytime.
The star in question is T Pyxidis, in the constellation of Pyxis, the Mariner's Compass. T Pyxidis is about 6,000 light years away and belongs to a small and seemingly exclusive group of cataclysmic variable stars called recurrent novae (NR). Astronomers have been patiently waiting for T Pyxidis's next outburst for more than 20 years.
Normally this star shines at magnitude 14: that's about a thousand times dimmer than the faintest star that can be perceived by most human eyes on a dark, clear night. But on five occasions, in 1890, 1902, 1920, 1944 and 1967, this star brightened dramatically to magnitudes between 6.5 and 7 (a 1,000-fold increase in brightness in the most extreme case) making T Pyxidis just bright enough to be glimpsed without any optical aid. These eruptions came at an average of just over 19 years apart, and the longest stretch of time between them was 24 years.
But this month marks 40 years since the last outburst.
It was back on Dec. 7, 1966 that the most recent eruption was first noticed by New Zealand amateur astronomer, Albert Jones. The star had more than doubled in brightness to magnitude 12.9. Just two nights later it was almost four magnitudes brighter and after a month it was glowing at magnitude 6.3 before slowly fading back to normal.
Nobody knows exactly why T Pyxidis has remained quiet for so long, but the general consensus is that it may have accumulated an extra-thick coating of nuclear fuel on its surface over these past 20 years, which would make it appear extra bright when it finally blows its next surge of gaseous debris out into space.
Who knows? That night could be tonight!
Analyse the text
- What are astronomers expecting to happen and to what?
- What is a nova?