Sky Observers' Glossary

For London and the UK

Telescope


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The Sun

The Sun rises roughly in the East and sets roughly in the West. During the year, the Sun's behaviour changes following the seasons.

From London (UK), the Spring Equinox occurs around 21 March. At this time, the length of daylight is 12 hours and the Sun reaches a noonday altitude of 38.5°. (This figure is 90° MINUS the Latitude of London, 51.5°).

Subsequently, the length of day increases, as does the noonday altitude. On the Summer Solstice (around 21 June) the length of daylight is 16 hours 38 minutes and the Sun's noonday altitude is 62°. (38.5° PLUS 23.5° - the tilt of the Earth's axis).

The days then get shorter. On the Autumn Equinox (around 23 September) the length of daylight is back to 12 hours and the Sun reaches the same noonday altitude as in March (38.5°).

The length of day continues to decrease. On the Winter Solstice (around 21 December) the length of daylight is only 7 hours 56 Minutes and the Sun's noonday altitude is a mere 15°. (38.5° MINUS 23.5° - the tilt of the Earth's axis).

The seasonal cycle continues as the length of the day and noonday altitude increase back to the values for the Spring Equinox.

The figures given here are also correct for places on the same latitude as London (UK). The exact dates of the equinoxes and solstices can vary by a day or so. For places closer to the Equator, noon-day altitude of the Sun is higher and the variation in day length throughout the year is less. For places closer to the North Pole, the noon-day altitude of the Sun is lower and the variation in day length throughout the year is more. In the Southern Hemisphere, the seasons are reversed.

The Sun appears to move around the Earth in a year. It blocks out different stars each month. Each month different stars are visible in the sky.

Twilight

If the Earth had no atmosphere, it would become dark as soon as the Sun disappeared below the horizon. The Earth's atmosphere disperses light from the Sun below the horizon causing it to get dark slowly. This is called twilight.

There are three types of twilight. During Civil Twilight, it is possible to read without artificial light. Civil Twilight ends when the Sun is 6° below the horizon. The bright stars and planets become visible at this time. During Nautical Twilight it is possible to see the horizon at sea for measurements to be made. This ends when the Sun is 12° below the horizon. At the end of Astronomical Twilight, even the faintest stars are visible. This ends with the Sun 18° below the horizon.

In June from London, Astronomical Twilight never ends since the Sun never reaches 18° below the horizon. Further North, there is a significant twilight during June (The White Nights).

Conjunction

Two objects close together in the same part of the sky. When the Moon is in conjunction with a planet, that is a good time to find the planet as it will be close to the Moon in the sky. Two planets can be in conjunction with each other. If the planets are bright a planetary conjunction can be a spectacular sight in the sky.

When a planet is in conjunction with the Sun, it will rise at sunrise and set at sunset; normally a planet is not visible when it is in conjunction with the Sun.

The Moon

The Moon travels around the Earth taking a month to do so. Its motion against the starry background can easily be seen from night to night. The Moon moves roughly its own diameter every hour.

The Moon is a dark body illuminated by the Sun. As the angle between Sun, Moon and Earth changes, the amount of the sunlit Moon visible from the Earth varies. This produces the Phases of the Moon: New, Crescent, Half, Gibbous, Full.

At New Moon, the Moon is in conjunction with the Sun is not usually visible (except during an eclipse). The New Moon rises and sets with the Sun.

Each day, the Moon moves eastwards from the Sun by about 13°. After a few days, a thin Crescent Moon will be visible in the evening setting soon after the Sun. As each day passes, the phase of the Moon increases (the crescent gets fatter - the Moon is said to waxing) and its time of setting gets later (about 50 minutes per day). Tides are mainly caused by the Moon - times of high tides also get later by 50 minutes each day.

The Half Moon occurs about 7 days after New Moon and is usually due south at sunset (from London, UK), setting around midnight. As the Moon continues waxing (and setting later) it displays a shape called the Gibbous Moon.

Two weeks after the New Moon, the Moon reaches opposition to the Sun (the Full Moon). The Full Moon rises at sunset and sets at sunrise, being visible all night long. In the summer (when the Sun is high) the Full Moon hangs low in the sky. In the winter (when the Sun is low) the Full Moon rides high in the sky.

After Full Moon, the Moon begins waning. The second half of the lunar cycle occurs later and later, mainly in the morning. The Half Moon that occurs three weeks after the New Moon rises at midnight and is due south at sunrise. Near the end of the cycle the Moon is a thin crescent rising shortly before the Sun.

The Moon's orbit around the Earth is not a circle but an ellipse. When it is at its closest to the Earth, it is said to be at perigee; when at its furthest from Earth, it is at apogee.

Eclipses

Eclipses of the Sun (also called Solar Eclipses) occur when the Moon covers the Sun. There are three types:

In a calendar year, there must be two solar eclipses and there can be as many as five.

Eclipses of the Moon (Lunar Eclipses) occur when the Moon (at full phase) passes into the shadow of the Earth. These eclipses can be total: the Full Moon slowly disappears to be replaced by ghostly reddish Moon for up to 1 hour 43 minutes. Partial eclipses are also possible.

Lunar eclipses are less common than solar eclipses. In a year none may occur; the maximum number is three. When lunar eclipses occur, however, they are visible from over half the Earth so they are easier to see. From a single location, several lunar eclipses will be seen per decade.

Planets

One of five star-like objects in the sky that are not fixed in position like the stars. They are Mercury, Venus, Mars, Jupiter and Saturn.

The word planet means wanderer in Greek. The planets move against the starry background because they are worlds close to the Earth that are moving around the Sun.

The five naked eye planets have been known since ancient times. From 1781, two more major planets have been discovered by telescope (Uranus and Neptune) and a number of minor planets (including Ceres, Pluto, Sedna).

Inferior Planets

The Inferior Planets are closer to the Sun than the Earth. Mercury and Venus are the Inferior Planets. An Inferior Planet can only been seen in the vicinity of the Sun; either in the evening after sunset or in the morning before sunrise. It can never be visible all night long.

An Inferior Planet can pass behind the Sun. This is called Superior Conjunction. It will not be visible at this point as it will be too close to the Sun in the sky. The planet then moves to the East of the Sun where it becomes visible in the evening sky after sunset. It is then misleadingly called an evening star. As an evening star, Mercury is normally visible for a few weeks before disappearing while Venus can be visible for several months at a time.

For observers in the Northern Hemisphere facing South, a planet that is East of the Sun is to the left of it, following it in the sky as it moves from left to right during the day. This is why the planet will be visible after sunset.

When the planet is as far from the Sun as it can be in the evening sky (as seen from the Earth), this is called Greatest Elongation East. Mercury can never be more than 28° away from the Sun (the amount varies because Mercury has a very eliptical orbit and can be as low as 18°). For Venus the greatest elongation is 47°.

After an evening appearance, the planet then moves in front of the Sun as seen from the Earth. This is called Inferior Conjunction. If the alignment is exact, the planet can actually pass in front of the Sun as a small black spot. This is called a Transit and is a rare phenomenon.

After Inferior Conjunction the planet moves West of the Sun into the morning sky (where it is called a morning star). The Greatest Elongation West is the farthest the planet can be from the Sun in the morning sky (as seen from Earth).

For observers in the Northern Hemisphere facing South, a planet that is West of the Sun is to the right of it, preceeding it in the sky as it moves from left to right during the day. This is why the planet will be visible before sunrise.

After a period of morning visibility, the planet moves back to Superior Conjunction and the whole cycle begins again. A complete cycle (say, from Superior Conjunction back to Superior Conjunction) is called the Synodic Period or Synodic Cycle.

Mercury's Synodic Cycle takes just under four months. The planet is at its brightest on either side of Superior Conjunction. For Venus, the cycle takes 16 months and the planet is at its brightest about a month on either side of Inferior Conjunction. The length of the Synodic Cycle depends on a planet's orbital period as well as the length of the year (the Earth's orbital period).

The Inclination of the Ecliptic

The visibility of a planet is determined by its Elongation from the Sun and something called the Inclination of the Ecliptic. The Ecliptic is the path of the Sun through the sky as seen from Earth. The Sun travels along it and the Moon and planets travel close to it. The Ecliptic runs through the familiar constellations of the Zodiac.

In high latitudes (like in the UK), the Inclination (or steepness) of the Ecliptic varies throughout the year. This affects the visibility of planets close to the Sun, especially the inferior planets.

Somethimes, the ecliptic makes a very shallow angle with the horizon. When this happens, planets will appear very low down after sunset or before sunrise. The period of visibility will be short: the planet will either rise very close to the time of sunrise, or set very close to the time of sunset.

At other times, the ecliptic makes a steep angle with the horizon. Planets will then appear much higher in the sky. The period of visibility will be long: the planet will either rise a long time before sunrise or set a long time after sunset.

In the case of Venus, this can make the difference between the planet making a spectacular appearance, high up or being barely visible close to a bright horizon. In the case of Mercury, a steep ecliptic means the planet will be visible, while a shallow ecliptic means that the planet cannot be seen.

In the Northern Hemisphere, the steepest ecliptic is found in the evening sky between February and April and in the morning sky between August and October. These are the best times to see the inferior planets: as evening objects in the late winter and early spring or as morning objects in the late summer or early autumn. If the planets have a good elongation from the Sun at these times, Venus will dominate the sky and be visible for several hours after sunset or before dawn; Mercury will be easily visible to the naked eye.

Conversely, the shallowest ecliptic is found in the morning sky between February and April and in the evening sky between August and October. These are the worst times to see the inferior planets: as morning objects in the late winter and early spring or as evening objects in the late summer or early autumn. Even if the planets have a good elongation from the Sun at these times, Venus will be visible but only briefly after sunset or before dawn and will be low down close to the horizon; Mercury will be unobservable.

The steepness of the ecliptic also effects the Moon. The best time to see the thinnest crescent Moon that occurs after New Moon is in the evening sky during the late winter or early spring. The effects of the ecliptic are shown in the diagram below which is after sunset.

Evening Planets (Northern Hemisphere)
In the diagram above, the Sun (yellow circle) is shown below the horizon (black line) at the end of Civil Twilight in the Northern Hemisphere. The Earth's Equator is the blue line. In the temperate regions, the Equator will be at an angle. The positions P1 and P2 are planets (grey circles) at the same elongation from the Sun. In the case of P1, the planet is south of the Sun and the ecliptic (red line) is at a shallow angle; the planet appears very low above the horizon when it begins to get dark. For P2, the planet is north of the Sun and the ecliptic (green line) is at a steep angle; the planet is much higher in the sky. The black dashed lines are the paths of the planets to reach the horizon as the Earth rotates. These paths are parallel to the Equator. For P1, the dashed line is the shorter meaning that the planet will reach the horizon very quickly; it will set in a short time. For P2, the dashed line is the longer meaning that the planet will take a long time to reach the horizon; it will remain visible for a long time before setting.

Transits

Only the Inferior Planets can transit the Sun. A transit occurs when an Inferior Planet passes directly between the Earth and the Sun appearing as a black spot against the Sun. Transits are rare.

Transits of Mercury occur about 13 times per century; nine in November and four in May. The transit is not visible to the naked eye and must be viewed by projection. The last one was in November 2006.

Transits of Venus only occur in June and December. A pair of transits is separated by 8 years but each pair occurs after 105 or 121 years. A transit of Venus can be seen with the naked eye if a filter is used. The dates of Venus transits below indicate how rare the event is. The last transits of Venus occurred in June 2004 and June 2012 but, before that, none occurred in the 20th Century.

Superior Planets

The Superior Planets are Mars, Jupiter and Saturn. These planets are further from the Sun than the Earth.

A Superior Planet cannot pass between the Earth and the Sun so there is no Inferior Conjunction. Since it can only go behind the Sun (Superior Conjunction) this is normally referred to, simply, as Conjunction. The planet will not be seen when it is in conjunction with the Sun.

The planet then moves to the West of the Sun. It appears in the morning sky. As the months pass, the planet will rise earlier and earlier as it moves away from the Sun in the sky, as seen from the Earth.

Eventually, the planet will be in Opposition. At Opposition, the planet rises at sunset and sets at sunrise and is visible all night long. A Superior Planet is at its closest to the Earth at Opposition and is then at its brightest. The elongation of a planet at opposition is 180°.

After Opposition, the planet will set earlier and earlier and will be seen in the evening sky. Eventually, the planet will set very shortly after the Sun and will then disappear in the evening twilight as it reaches Conjunction again.

A complete cycle (say, from Opposition back to Opposition) is called the Synodic Period or Synodic Cycle.

For Mars, the Synodic Cycle takes 26 months. So Mars is prominent in the sky every alternate year. For Jupiter the cycle takes just over 13 months and so occurs a month later each year. For Saturn, the cycle is just under 54 weeks.

There are two other superior planets. Uranus can just about be seen with the naked eye on a dark Moonless night. A small telescope or a good pair of binocculars are required to see Neptune. Both these planets travel very slowly around the sky and have Synodic Periods just over a year long.

The Altitude of the Ecliptic

The Ecliptic is essentially the path of the Sun in the sky over a year. In June the Sun is very high in the sky and the days are long. In December the Sun is very low in the sky and the days are short.

The planets also follow the ecliptic. This means that their altitudes in the sky and their periods above the horizon vary depending on which part of the ecliptic they are in.

The best time to observe the superior planets is when they are at opposition. They are then opposite the Sun. If the Sun is high in the sky, a planet in opposition will be low. If the Sun is low in the sky, a planet in opposition will be high.

It follows that the best time of the year to see a planet in opposition is December. At that time the Sun will be low in the sky and the days will be short. A planet in opposition will be high in the sky and the nights are long. In June, a planet in opposition will be low in the sky and the nights will be short.

The Moon is also affected by the altitude of the ecliptic. The Full Moon is essentially a Moon in opposition. This explains why the winter Full Moon is high in the sky while the summer Full Moon is low in the sky.

Occultations

An occultation occurs when one object covers or occults another. The Moon can occult four bright stars: Aldebaran (Taurus), Regulus (Leo), Spica (Virgo) and Antares (Scorpius). These occultations occur at irregular intervals. The Moon can also occult planets. These occultations are even rarer. Rarest of all is an occultation of a star by a planet and one planet occulting another.

Stars

Stars are distant suns. They vary in brightness and colour. Human beings have connected unrelated stars into groups called Constellations. Modern astronomers recognise 88 Constellations.

Different cultures (Mayan, Chinese) have used different groupings. Of the modern Constellations, 48 are from Greek and Roman times. These 48 constellations (mostly in the Northern sky) are known as the Classical Constellations. They include the 12 constellations (also known as signs) of the Zodiac (e.g. Scorpius, Cancer, Leo) in which the planets, Sun and Moon are always found, and other famous names like Orion, Ursa Major (the Great Bear), and Hercules.

When Europeans began exploring the Southern Hemishere, new stars were discovered and 40 new Constellations were created. These include Crux Australis (The Southern Cross), Centaurus and Carina.

The Milky Way is a collection of stars too numerous to be seen individually. To see it, a dark moonless night away from city street lights is required. The best time of year is Winter or Summer.

Many stars when viewed through a telescope turn out to be doubles - two stars forming a single system and in orbit around each other. These are called Binaries. Albireo in Cygnus is an easy example to see and the two stars are different colours. In some cases the two stars are not related and only happen to be in the same direction in the sky. These are Optical Doubles. Mizor (and Alcor) in the Great Bear is the best known example.

Some stars are bunched together over a small area of the sky. These are called Star Clusters. There are two types.

The more scattered are called Open Clusters. The most famous in the Northern hemisphere sky is The Pleiades (also known as The Seven Sisters) in Taurus. This is easilly visible with the naked eye and is superb in binocculars. Others include Praesepe (the Beehive) in Cancer and the Double Cluster in Perseus.

Spherical and more bunched together are the Globular Clusters. The brightest in the Northern Hemisphere is the Great Cluster in Hercules (known by astronomers as M13). The best ones are only visible in the Southern Hemisphere.

Some stars vary in brightness and are called Variable Stars. Some are actually changing the amount of light they emit (Intrinsic Variables). The easiest to see is Mira (the Wonderful) in Cetus. This star varies over about 330 days from being as bright as the stars in the Great Bear to being below naked eye visibility. Most variable stars are not this obvious. Another group of stars vary because they are really a double and one member eclipses the other. These are Eclising Variables. The best known is Algol (the Demon star) in Perseus which appears to wink for an hour or so every few days. Explosive Variables are just that - exploding stars. They are unpredictable and rare. They have been known to outshine Venus in the past.

Apart from the stars there are more nebulous objects visible in the sky. A Nebula is a interstellar gas cloud. The easiest to see is the Orion Nebula (visible in Winter in Orion). A Galaxy is a fuzzy patch that is actually a very distant system of stars, clusters and nebulae. The Andromeda Galaxy (Autumn in Andromeda) is the furthest object visible to the naked eye.

Meteors

Meteors (also called shooting stars) are small particles (usually smaller than a grain of sand) that burn up when they enter the Earth's atmosphere leaving a brief streak of light. On any dark night 10 to 15 can be seen every hour. The numbers increase after midnight.

At some times of the year, the Earth passes through a stream of particles and we can see a meteor shower. The numbers of shooting stars is then higher, occasionally very much more. These meteor showers appear to come from a particular region of the sky and are named after the constellation in that region. The most regular and famous meteor showers are the Perseids (apparently from Persus) every August and the Leonids (Leo) in November.

Shooting stars are nothing to do with stars.

Some particles are so large that they reach the Earth's surface. They are then called Meteorites.

Comets

Comets are small bodies compared to planets. They are covered with ice and travel around the Sun in (usually) non-circular orbits.

When they are a long way from the Sun they are not visible to the naked eye. As they approach the Sun, the radiation and gases from the Sun evaporate the ices and a tail appears. The tail always points away from the Sun. cometary tails may be millions of kilometres long but they are made up of very thin gases.

Comets do not streak across the sky. They hang amongst the stars, changing position night after night. The head of a Comet is fuzzy. Comets appear at irregular intervals. Naked eye Comets are rare and can be spectacular.

Asteroids or Minor Planets

These are bodies in the solar system that are orbiting the sun and are smaller than planets.

Many go around the Sun between the orbits of Mars and Jupiter. Ceries is the largest of these and is visible with binocculars. The brightest asterod is Vesta which can be just about visible to the naked eye at opposition. Most of the others require telescopes to be visible.

Other asteroids orbit among the inner planets, like Eros, which passes close to the Earth and Icarus which moves closer to the Sun than Mercury.

A small group follow in Jupiter's orbit. These are called Trojans and the first descovered was Achilles.

Still others orbit amongst the outer planets. These are called Centaurs and the best known is Chiron (not to be confused with Pluto's moon, Charon).

Beyond the planet Neptune are a number of smaller icy bodies, the best known of which is Pluto, but others include Sedna and Eris.

Polar Lights

Polar Lights are coloured glows in the upper atmosphere caused by the interaction of the gas molecules with charged particles from the Sun. These particles are diverted into the polar regions by the Earth's magnetic field. In the Northern Hemisphere they are called the Northern Lights (or Aurora Borealis). They are not normally visible as far south as the UK but displays have been known at these latitudes.


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