The Celestial Sphere

The Sun and stars show regular patterns of motion that reflect the rotation of the Earth around its North- South axis and the revolution of the Earth around the Sun. The motions show how the Solar System works — that indeed, the heliocentric “sun-centered” is true.

FRAMES OF REFERENCE: An observer can see half the celestial sphere at any given time, or 6 of 12 constellations. On Earth at any given time, different observers see different parts of the sky, and different motions of starts in their sky. This proves that Earth is a sphere that rotates once per day.

Twelve Constellations of the Ecliptic

CONSTELLATIONS: Constellations are groups of stars forming a pattern or an outline, such as Scorpius (Scorpion), Ursa Major (The Bear), Cygnus (The Swan), and Orion (The Hunter). Constellations in the circumpolar zone are close to the North Celestial Pole and stay up all night and year round, while those in the equatorial zone are near the Celestial Equator and changes during the night and year. An asterism is a sub-group of a constellation, such as the Big Dipper in Ursa Major. The twelve constellations of the zodiac (of the ecliptic) appear during certain months of the year: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.

The Celestial Sphere

THE NIGHT SKY: While stars “twinkle,” planets do not; planets only reflect light from stars. Stars appear to twinkle because light passes through Earth’s atmosphere and different pockets of air at different temperatures.

MAGNITUDE: In the original magnitude system, 1 is the brightest visible and 6 is the faintest visible. The difference of 5 magnitudes (6-1=5) is actually a factor of 100 in magnitudes. A magnitude 6 star is 100 times fainter than a magnitude 1 star. The modern system differs from the original one, with some stars brighter than magnitude 1 stars and others fainter than magnitude 6 stars. Human eyes respond to light logarithmically: m= -2.5log(b), where b = brightness of star by counting the number of photons per second.

ANGULAR MOVEMENTS & MEASURING WITHOUT A TELESCOPE: The width of your finger held at arm’s length equals 1 degree, while the width of your fist equals 10 degrees (e.g. full moon is about the width of half a finger, or 0.5 degrees).

1 revolution = 360°

2∏ radians = 360°

1° = 60 arcminutes = 60′

1′ = 60 arcseconds = 60”

1° = 60′ x 60°/1′

1° = 3,600”


3 thoughts on “The Celestial Sphere

  1. I didn’t understand your explanation of why planets don’t twinkle. You explain that ‘Stars appear to twinkle because light passes through Earth’s atmosphere and different pockets of air at different temperatures.’, though that is the very same thing that happens when we are able to see planets; the light reflected by them pass through our atmosphere. Why is it that they don’t twinkle? Surely, that light is subject to the same conditions that light from stars are.

    Thank you for your informative blog.

    • Thanks for your question. Planets twinkle as well, but not nearly as much so that we can see it. Simply, planets are too close for the reflected light to be severely altered by our atmosphere. It is much easier for stars’ light to be distorted because they are literally small pinpoints of light, while planets are more like disks.


      • Thank you for explaining; that does make sense. Any twinkling is, as I understand you, cancelled by there being an effectively (from our perspective) larger surface ‘sending out light’ – a parallax effect, I presume.

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