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Friday, October 20, 2017

Observing Objects

One of the first things to notice with preliminary studies of the sky is that stars and other objects are of differing brightness. The easiest way to determine what you are observing is to take advantage of this. With the exception of the Moon, the brightest objects in the night sky are (some of) the planets. The planets change their positions from night to night in respect to the background constellations of stars. There are three main characteristics distinguishing a planet from a star: location, brightness and twinkling.


The planets always travel within a few degrees of the path of the Sun. The path that the Sun appears to travel against the sky is called the ecliptic. It marks the centre of the band of sky within which the Moon and planets are found. This area is known as the zodiac. The ecliptic passes through the 13 zodiacal constellations of Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Ophiuchus, Sagittarius, Capricorn and Aquarius. Ophiuchus (The Serpent Bearer) does not appear in the astrologer’s Zodiac although the planets spend more time in it than in Scorpio.

Astrologers use a different zodiac from astronomers for reasons of precession. The Earth moves on its axis with a wobbling motion like a spinning top, the axis tilted away from the vertical by 23.5 degrees. Whereas the axis of a top takes only a few seconds to complete its reeling movement, the period for the earth is 25,800 years. This movement, known as precession, causes slow changes in which constellations make up the zodiac. Astronomers use the real time zodiac, whereas astrologers use the zodiac of more than 2,000 years ago.

Location is a big clue as to whether you might be observing a planet (or not). If you know where the Sun and Moon rise and set you can follow this line across the sky, night or day, and observe the planets in our Solar System following a similar route.


Some of the planets quite simply appear too bright to be stars. Venus, the brightest planet, is an example. It is never far away from the Sun in the sky, so whenever an extremely bright object appears in the sky towards the west after sunset, or in the morning towards the east before sunrise, most probably it is Venus.

As an evening object, Venus is often the first bright object visible, before any stars appear in the sky. Mercury also appears in areas of the sky around sunrise and sunset but never looks as bright as Venus nor is as far from the Sun as Venus. Mercury appears only during twilight, and Venus never remains visible through the night. Whenever a very bright yellowish white point of light appears in the sky in the middle of the night, it is probably Jupiter. Unlike Mercury and Venus, Jupiter is not always near the Sun in the sky and can appear high in the sky at midnight. Mars and Saturn can also appear far from the sun in the sky, rising well after sunset. Mars rarely out-shines Jupiter and the brightness of Saturn never equals that of Jupiter or Venus. Mars can often be distinguished by the fact that it has a slight but distinct reddish or orange colour. Saturn, on the other hand, appears to be yellowish. The other planets are too faint to be seen with the naked eye.


As per the nursery rhythm, stars twinkle. Planets, however, usually seem to shine steadily. Twinkling is an effect of turbulence in the Earth’s atmosphere whereby starlight passing through the atmosphere shows the intensity to vary slightly but rapidly. Observations with a telescope reveal that a star appears to slightly move about. The reason why stars twinkle and planets do not is that stars are so far away that they look like points of light even when viewed through large telescopes but planets are close enough to Earth to be seen as a disc by telescopic image. The light from different parts of a planet’s disk averages out and makes a planet appear relatively steady in both brightness and position. Planets may seem to twinkle if the atmosphere is especially turbulent, or if an object is low in the sky (observed through large amounts of atmosphere). Under these conditions, an object may seem to change in colour – i.e. when Venus is low in the western sky it can change from a greenish hue to a reddish hue and back again.

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