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Background Information for Astronomy

Background Information for Astronomy

Below you will find a few subjects of Astronomy to investigate.

Telescopes - The Astronomers' Tools

Optical Telescopes: With these telescopes, light is either collected by a lens system as in the Museum's refracting telescope or by mirrors as in a reflecting telescope, or a combination of both.

1) Refracting telescopes use lenses which are precisely curved pieces of glass that collect light. When light passes through a lens, its direction is "bent" (or refracted). Lenses can be shaped so that all light rays passing through are bent to converge at a point called the focus, where an image is formed. The image is then magnified by the eyepiece and viewed by the observer.

Refraction occurs when light passes from one medium to another medium. To give a concrete example, ask students to try this activity at home or at school. Put a pencil in a glass half filled with water.

"What happens to the pencil?"

It appears bent because light is refracted; light traveled from one medium - the air, and passed through another medium - the water.

A refracting telescope

A refracting telescope - Note the light path and the inverted image.
The focal length is the distance between the objective and the focus.

2) Reflecting telescopes use curved or concave (curved inwards) mirrors to focus light. The light from a distant object falls on the primary mirror (also called the objective) and is then reflected to the secondary mirror. The secondary mirror then reflects the light to a focus, a convenient location for the image to be observed where it is then magnified by the eyepiece.

A reflecting telescope

A reflecting telescope - An example of the light path in a Newtonian type reflecting telescope.

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The Museum's Telescope

The Helen Sawyer Hogg Observatory

The Museum's main telescope, which is located in the Helen Sawyer Hogg Observatory, is the 38 cm (15 inch) refracting telescope from the old Dominion Observatory. It is the largest refracting telescope in Canada. Officially opened in 1905, the Dominion Observatory is a familiar landmark, and many Canadians, young and old, looked through the telescope from that site. When all Canadian Government astronomy was consolidated in the National Research Council in 1970, the telescope was not transferred but remained in the dome of the Observatory building where, with the support of this Museum, it was used in a public education program until July, 1974. It was then disassembled and brought to the Museum where it was refurbished and installed in the Museum's observatory in January, 1975. In September 1989, the observatory was named after Helen Sawyer Hogg, a renown Canadian astronomer who popularized astronomy.

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The Sun - the Earth's Star


The Sun is not a solid body; it is a giant ball of gas - mostly hydrogen and helium. Energy is produced deep within its core (by means of nuclear fusion). Because of its relative proximity, astronomers can gain important knowledge about other stars by studying our Sun, for it is the only star close enough to display a disk. Also, stress the Sun's importance in our lives. The Sun is responsible for all life on Earth, it gives us light and heat.

In size, our Sun is not the largest, nor is it the smallest star. It is just average. The largest stars are called supergiants and the smallest are called dwarfs. Our Sun is a dwarf star. The star Betelgeuse located in the constellation of Orion is a supergiant. If we were to put it where the Sun is, it would be so big that it would engulf the orbit of Mars. Betelgeuse is about 800 times larger than the Sun.

In terms of brightness (luminosity) the Sun is also an average star. It appears bright to us because it is so close (close in astronomical terms - compared to other stars). However other stars can be much brighter or much fainter than our Sun.

Our Sun is very hot. If a spaceship traveled towards the Sun it would burn up and melt long before getting there. A star's temperature varies with depth, i.e. the temperatures are much higher at the core (14,000,000 C) than they are at the surface (5,800 C).

The Sun is considered to be a middle aged star approximately 4.6 billion years old. Like humans, stars have a lifespan. They are born from interstellar dust and gas, they reach stellar "adulthood", then middle age (where they spend most of their lifetime), and then they die, sometimes in spectacular ways, i.e. novae, black holes. Our Sun appears to be halfway between birth and death; it is middle-aged.

The Sun

The Sun is by far the most massive object in our Solar System, it is about 333,000 times more massive than the Earth. 109 Earths side by side would fit inside the (equatorial diameter) of the Sun.

The Sun does not rotate as a solid body because of its gaseous composition. The period of rotation varies from 25 to 35 days depending on latitude.

Website: N.A.S.A. solar Photo Gallery

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Solar Phenomena

The Sun is not a quiet ball of gas but is very active. It exhibits certain types of activity that are observable over a period of time. These phenomena vary in an 11 year cycle called the solar cycle. They include sunspots, flares, prominences and the solar wind.

Sunspots are dark markings on the Sun's photosphere. They appear dark because they are cooler than the surrounding area. The number of sunspots reaches a maximum and a minimum over an 11 year cycle called the sunspot cycle. Sunspots generally occur in pairs and are regions of strong magnetic fields.

Solar flares are explosive eruptions of gas that occur at the site of a sunspot. They can extend beyond the corona. Flares are short-lived solar activity that last several minutes and they are much hotter than the Sun's surface. A solar flare may cause aurora on Earth, interfere with radio transmissions and even cause power outages.

Prominences are huge eruptions of gas that are ejected up to thousands or even hundreds of thousands of kilometers above the Sun's surface and fall back as arches. Like sunspots, prominences are cooler than the solar surface and are longer lived than flares. They last on the order of weeks or months.

The solar wind is a flow of invisible energetic charged particles flowing from the Sun.

Image of Sun from N.A.S.A. (32 KB)

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The Earth

Earth is the third planet from the Sun and the only known planet to support life. The earth is massive enough to support an atmosphere, which is a layer or envelope of gas that surrounds a planet or other body.

A few characteristics of the Earth's atmosphere: 

  • it is made of gases (78% nitrogen, 21% oxygen, and 1% other gases) that support life; it allows us to breath;
  • it protects us from the Sun's harmful effects;
  • it keeps us at a relatively warm temperature;
  • all of the weather occurring on Earth is created in the atmosphere i.e. wind, hurricanes, clouds, lightning, snow and rain storms;
  • energy from the solar wind and solar flares reach Earth and interact with gases in the upper atmosphere to produce aurora. They are called aurora borealis, or northern lights in the northern hemisphere and aurora australis in the southern hemisphere.

Earth's surface features: 

  • it is a solid surface;
  • it is the only known planet to support life;
  • 2/3 of the surface of the Earth is covered with water;
  • the Earth's surface has certain features that can be found on other planets; it has mountains, valleys, volcanoes, geysers, earthquakes and craters.

Picture of Earth from N.A.S.A. (74 KB)

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The Seasons

The seasons occur because the Earth's rotation axis is tilted by 23.50 relative to the plane of its orbit around the Sun. This causes the Sun to be higher at noon in our summer than in winter. Note that the seasons are not produced by the distance between Earth and Sun. In fact, the Earth is closest to the Sun in the winter than in the summer. This is brought about by the elliptical shape of Earth's orbit.

The Seasons

When the Earth's northern hemisphere is tilted towards the Sun, it is summer. It begins on, or about June 21st , the summer solstice. It is the longest day of the year. The Earth's northern hemisphere tilts away from the Sun in winter. Winter begins on or about December 21st and is called the winter solstice. It is this day when the Earth's axis is tipped furthest from the Sun, causing it to be the shortest day of the year. Temperatures are warmer in summer than in winter because the Sun is higher overhead, causing more direct heating of the surface. In the southern hemisphere winter would begin. The Sun is lower in the sky and its rays are spread over a larger area, causing a less efficient heating of the surface.

Seasons: Summer
Seasons: Winter

For those living in the northern hemisphere, when Earth reaches a point where the hours of daylight equal the hours of night, autumn begins. This is called the autumnal equinox. It occurs on or about September 21st . In the southern hemisphere, spring begins. Six months later, when the Sun and Earth are in such a position that the hours of daylight again equal the hours of night, spring begins in the northern hemisphere. This is called the vernal equinox and occurs on or about March 21st. For those in the southern hemisphere, autumn begins. For those living on the equator, the hours of daylight and the hours of darkness are always equal.

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The Moon

The Moon shines by reflected sunlight. It is the second brightest heavenly body after the Sun and the only Solar System body to be visited by humans. On July 20, 1969, the Apollo 11 mission landed 2 humans on the Moon while a third (Michael Collins) waited in orbit in the command module Columbia. Neil Armstrong (1st) and Edwin "Buzz" Aldrin became the first people to walk on another world after their lunar module, Eagle, landed in the Sea of Tranquillity.

Our satellite, the Moon, is about 1/81 as massive as Earth and has 1/6 the Earth's gravity. If you weigh 54kg on Earth you will weigh only 9kg on the Moon. This is the reason why the Apollo astronauts were able to "walk" on the Moon by leaps and bounds. The Moon has no atmosphere because of its weak gravity; it is not strong enough to hold an atmosphere. There has been a recent discovery of ice at the North pole of the Moon.

When looking at the Moon with the unaided eye, two types of terrain can be noticed. The dark markings on the Moon are called mare, pronounced mar-eh, (maria -plural) meaning "sea" in Latin, and the lighter areas (the highlands) are regions that are heavily covered with craters.


Long ago, about the time that Galileo first pointed his telescope towards the Moon in 1610, some astronomers thought that the dark areas on the Moon were seas filled with water. We now know that this is not the case, however, the word "sea" or mare has been kept. The seas on the Moon are in fact relatively smooth areas caused by lava flows. This lava covered large regions of the Moon billions of years ago, covering and filling in many craters. The lava does not reflect light well and so appears dark to an observer on Earth.


The lighter colored regions on the Moon (also called the highlands) are covered with craters. Galileo first discovered these features when he pointed his telescope to the Moon. Craters are circular depressions caused by the impact of meteorites. Some craters on the Moon were caused by volcanoes, however, lunar craters caused by meteorites far outnumber those caused by volcanic activity. Craters on the Moon can vary in size from less than one metre in diameter up to 1,000 km in diameter.


  • Asimov, I. Guide to Earth and Space. New York: Random House, 1991.
  • Patrick Kelly, ed. Observer's Handbook, 2008, Toronto: University of Toronto Press, 2008. (published annually by the Royal Astronomical Society of Canada)
  • Enright, Leo. ed.The Beginner's Observing Guide. Toronto: Royal Astronomical society of Canada, 1996.
  • Illingworth, V.The Facts on File Dictionary of Astronomy. New York: Facts on File Publications, 1985.
  • Mary Lou Whitehorne, Skyways - Astronomy Handbook for Teachers, Royal Astronomical Society of Canada, 2003.

Website: N.A.S.A. Photo Gallery of the moon

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