Astronomy for GCSE







Astronomy... 4. The Earth as a Planet

Patrick Moore begins this chapter by stating that while the earth is of course important to us... it's a very ordinary planet.

The size is normal enough:
It has a diameter (the book claims) or 12,756 km as measured through the equator, but only 12,709 km as measured through the poles, so that it is what is technically as an oblate spheroid, rather than a perfect sphere.

It's hard to corroborate those dimensions with Wikipedia as there they are given as radii:
Equatorial radius: 6,378.137 km
Polar radius: 6,356.752 km

I do wonder if there is a discrepancy (the latter gives a diameter of 12,713.504) due to measurements being updated through a more accurate means.

Perhaps the Wikipedia page on the World Geodetic System (WGS) has the answer:

[WGS] is a standard for use in cartography, geodesy, and satellite navigation including GPS. This standard includes the definition of the coordinate system's fundamental and derived constants, the normal gravity Earth Gravitational Model (EGM), a description of the associated World Magnetic Model (WMM), and a current list of local datum transformations...

All components of WGS84 [the present version] are regularly updated. ... Its Definition and Relationships With Local Geodetic Systems document, initially published in 1984, has been revised in 1997, in 2004 ... and finally in 2014 ... The regularly-updated documents provide refined descriptions of the Earth and realizations of the system for higher precision.

The bulging at the equator is because the earth is rotating on an axis.

For comparison, Mars has the following dimensions (according to Wikipedia):

Equatorial radius: 3,389.5
Polar radius: 3,376.2

The orbit around the sun is also not circular; as Johannes Kepler showed, it is an ellipse, with the sun lying in one focus.

The ellipse produced by the orbit actually has two foci.

Kepler, under the direction of Tycho Brahe, was trying to understand and calculate the orbit of Mars. Things weren't adding up with either a circular or egg-shaped orbiit. Then in late 1604 he at last hit upon the idea of an ellipse, which he had previously assumed to be too simple a solution for earlier astronomers to have overlooked. Finding that an elliptical orbit fit the mars data, [he] immediately concluded that all planets move in ellipses, with the Sun at one focus - his first law of planetary motion.

When Earth is closest to the Sun (perihelion) the separating distance is 147,000,000 km, at its furthest (aphelion) it is 152,000,000 km, giving a mean of 149,598,500 km, or one astronomical unit (AU).

You might think that winter occurs when the earth is closes to the sun, but you'd be mistaken. The seasons are due to the tilt of the earth's axis [link].

It takes the Earth 365.256 days (rotations on its axis) to complete one orbit of the Sun. This is slightly longer than our calendar year, which is why we have Leap Years where February includes an extra day every four years.

The earth's atmosphere is made up of 78% nitrogen and 21% oxygen. The remaining 1% includes other gasses such as argon and carbon dioxide. The atmosphere extends upward for hundreds of kilometers, but most of it is concentrated in the lowest layer, known as the troposphere. This layer lies between sea-level and a mean height of 8 km over the poles and 11 km over the tropics. This is where we find all our 'weather', and all ordinary clouds. There is very little trace of atmosphere left above a height of 1000 km.

The atmosphere is unsteady and, according to Patrick, this is what makes the stars twinkle. However, this doesn't seem like a satisfactory explanation unless the visible planets are within our own atmosphere! says this:

Stars twinkle because … they’re so far away from Earth that, even through large telescopes, they appear only as pinpoints. And it’s easy for Earth’s atmosphere to disturb the pinpoint light of a star. ... Planets shine more steadily because … they’re closer to Earth and so appear not as pinpoints, but as tiny disks in our sky.

Water vapour in the high part of the troposphere may split up the sunlight and cause rainbows.

Sometimes a thin layer of ice-crystal cloud will make the Sun and the Moon produce a halo.

It is the atmosphere which scatters the blue part of the Sun's light around to make the sky blue. When the sun is low down, more of its red rays can pass unchecked.

"In astronomy the term 'day' is taken to mean the whole 24-hour period, not just he interval between sunrise and sunset." Coincidentally, in reading The Book of Ceremonial, some prayers are given with the instruction that they are to be said during the first hour of the day; does it mean immediately following sunrise?

The "march of the seasons... have very little to do with the Earth's changing distance from the Sun, and we are actually at our closest to the Sun in December, when it is winter in Britain... [It is the tilt of the] Earth's axis... the angle between the axis and the perpendicular is 23o26', or approx. 23.5o. In June, the Earth's north pole is tilted toward the Sun, and the northern hemisphere receives the full benefit of the... rays... By December the conditions have been reversed."

When we want to give the position of any point on the Earth's surface, we do so by quoting its latitude and longitude.

Latitude is simply the angular distance of the point from the Earth's equator reckoned from the centre of the globe (the latitude of the equator being 0o)

Longitude is the angular distance of any point east or west of the Greenwich meridian.

To understand what a meridian is we use the term great circle.

The Greenwich meridian [the Prime Meridian] is the great circle on the Earth's surface which passes through both poles and also through Greenwich. Longitude is measured up to 180o east and west. The longitude line on the opposite side of the world runs largely through the Pacific Ocean, missing New Zealand by a fairly wide margin. This is also known as the International Date Line.

Geographic poles are not the same as the magnetic poles; magnetic compasses do not indicate true north. [Indeed, the magnetic pole shifts]

From the book The Atlantis Blueprint by Rand Flem-Ath and Colin Wilson (which I read in 2014):

In October 1884, Professor Charles Piazzi Smyth, the Astronomer Royal for Scotland, was involved in a controversy on a matter dear to his heart: persuading a committee of experts from twenty-five countries of the world to make the north–south line that ran through the Great Pyramid the prime meridian of the world, 0 degrees longitude.

It may sound odd that, towards the end of the nineteenth century, when great steamships had been plying the oceans for decades, such a question should remain undecided. There had been numerous prime meridians – virtually one for every country that used the sea... Charles II of England decide[d] to build the Greenwich Observatory, with the intention of designating Greenwich as the prime meridian. The French disagreed, and then said it should run through Paris. As other countries built observatories, most of them declared their own capital the site of the prime meridian, which is why, in October 1884, twenty-five European countries gathered in Washington, DC, to make a final decision.

Greenwich was high on the list of candidates because so many ships used the port of London, but Smyth was passionately opposed to it. In the Report of the Committee on Standard Time and Prime Meridian,1 published in Cleveland, Ohio, in June 1884, he argued that the Pyramid was the ideal choice because such a meridian would pass over more land than any other.

The Great Pyramid, he pointed out, was acknowledged to be the grandest monument ever erected. [He also recognised that the Great Pyramid was located at the centre of the earth’s land mass.] As a further argument, he drew attention to its closeness to Jerusalem, evoked the Second Coming of Christ, and asked whether every good Christian would not agree that a Giza meridian would be ideal.

The answer was no. The delegates were not at the conference as Christians but as scientists.

[When Giza is used as the prime meridian, as it perhaps was in the ancient past, suddenly dozens of sacred sites (Ballbek, Paracas, Cuzco, Sidon, Machu Piccu, Ehdin, Ollantaytambo, Nineveh) ... fit into a vast global pattern. It is also said that into the Great Pyramid are programmed the dimensions of the Earth and it's place upon it.]

"Venus and Mars have no detectable [magnetic] fields... [although] the giant planets have very strong [magnetic] fields indeed." The following table is adapted from

Planet mag. field (* Earth's)
Mercury 0.006
Venus 0.00
Earth 1.000
Mars 0.00
Jupiter 19,519
Saturn 578
Uranus 47.9
Neptune 27

magnetic field (mag. field) is the total strength (NSSDC gives strength in #gauss × Rplanet3,
where Rplanet is the radius of the planet and Earth's strength = 0.3076 gauss × RE3 = 7.981×1010 gauss...

Because the Earth spins on its axis from west to east, the bodies in the sky seem to move from east to west, taking approximately 24 hours to complete a full circuit. Yet there are two points in the sky which do not seem to move at all; these are the celestial poles, which lie in the direction of the Earth's axis. The north celestial pole is marked fairly closely by the bright star which we call Polaris, or the Pole Star, in the constellation of Ursa Minor, the Little Bear. There is no bright star close to the south celestial pole, and we have to make do with the obscure star Sigma Octanis, which is barely visible to the naked eye...

Although appearing to the naked eye as a single point of light, Polaris is a triple star system, composed of the primary, a yellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab; the pair is in a wider orbit with Polaris B. The outer pair AB were discovered in August 1779 by William Herschel. - Wikipedia


More coming soon...

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