words/phrases used in observing, explained..
The ZHR or Zenithal Hourly Rate is the number of meteors an observer would see
in one hour under a clear, dark sky with a limiting apparent magnitude of 6.5 and if the radiant of the shower were in the
The rate that can effectively be seen is nearly always lower and decreases as the radiant is closer to the horizon. The Zenith is the
overhead point in the sky.
The radiant is the point in the sky, from which (to a planetary observer)
meteors appear to originate, i.e. the Perseids, for example, are meteors which appear to come from a point within the constellation of
When the radiant is quoted as “circumpolar”, it is never below the horizon and visible all night, otherwise the times quoted are when the
constellation in which the radiant lies rises above the horizon in the East.
A fireball is defined by the International Astronomical Union as a meteor
brighter than any of the planets, i.e. magnitude -4 or brighter. The International Meteor Organisation alternatively defines it as a meteor
would have a magnitude of -3 or brighter at the zenith.
The ° symbol in the guide is that for degrees. A degree is two full moon widths
to give an idea for judging any distances quoted in the guide.
An asterism is a collection of stars seen in Earth's sky which form simple
patterns which are easy to identify, i.e. the Big Dipper. They can be formed from stars within the same constellation or by stars from
more than one constellation. Like the constellations, they are a line of sight phenomenon and the stars
whilst visible in the same
general direction, are not physically related and are often at significantly different distances from Earth.
Mag is short for magnitude which is the measure of an object’s brightness. The
smaller the number, the brighter the object. The brightest object in the sky is the Sun at mag -26, the full moon is mag -12 and
Venus the brightest planet is mag -4. The brightest stars are mag -1. If there is a 1 mag difference
between two objects – there is a
difference in brightness of a factor of 2.5 between the two objects. For example the full moon is eight magnitudes brighter than Venus on
average which means it is 1,526 times brighter than Venus. Objects down to mag +6 can be seen with the naked eye under very dark skies.
Local time is always quoted in the guide and this means for November –
February – universal time (UT)/GMT is used and for April to September – daylight savings time (DST, = GMT+1). For the months
of March and
October when the clocks go forward/back respectively, both times will be used and
attention should be paid to any times
at the end of these months for that change.
Deep Sky Objects such as galaxies, nebulae and star clusters are classified in
catalogues such as the Messier catalogue for objects like M44 – M for Messier. Another example of a catalogue would the New
catalogue whose objects have the prefix NGC.
From Earth - Mercury and Venus are the inner planets in the solar system and
Mars, Jupiter, Saturn, Uranus and Neptune are the outer planets. Below is a short guide as to how both the inner and outer
move around the sun. The above pictorial guide should hopefully help in this.
The Inner Planets
These are best seen when at Greatest Eastern/Western elongation and are not
visible when at either Inferior/Superior conjunction. Greatest Eastern elongation is when the inner planet is at its furthest point
east from the sun as seen from Earth and visible in the evening sky in the West after sunset, Western
elongation is when its at its
furthest point west from the sun as seen from Earth and visible in the morning
sky in the East before sunset. Inferior conjunction
occurs when the inner planet is between the Sun and the Earth. Superior conjunction occurs when the inner planet is on the other side of the
Sun as seen from Earth.
From our Northerly latitudes, the ecliptic, along which the planets move, lies
at a very shallow angle to the horizon after sunset in t the autumn and before sunrise in the spring. This means that any of the planets will
be difficulto see when fairly close to the Sun in the evening sky in the autumn, or in the morning sky in the
spring. In particular,
Mercury is more or less invisible from here when at Eastern elongation in the autumn,
or at Western elongation in the spring, because it
lies so close to the horizon and is never above the horizon except in daylight or bright twilight.
The normal cycle for an inner planet is Superior Conjunction – Greatest
Eastern Elongation – Inferior Conjunction – Greatest Western Elongation - Superior Conjunction. After superior conjunction, the
planet moves away from the Sun as seen from Earth and becomes visible in the evening sky after a period of time. It then moves
past the point of Greatest Eastern Elongation and moves back towards the Sun as seen from Earth until a point when it is not visible and
at Inferior Conjunction. After this the planet appears in the morning sky for a time, before again slipping into the Sun’s glare as
seen from Earth. The duration of this cycle will depend on the planet’s closeness to the Sun, i.e. Mercury completes the above cycle in
around 4 months.
The Outer Planets
These are best seen when at opposition and are not visible when at conjunction.
Opposition occurs when the earth is between the sun and the outer planet. It is the best time to observe them because the planet is
visible all through the night and it is due South and at its highest at about midnight. The planet is also at its closest point in its orbit
to Earth – making it appear brighter. Conjunction occurs when the outer planet is on the other side of the Sun as seen from Earth.If the planet is at or near it furthest point South along the ecliptic, then it
won’t get very high in the sky even at opposition – just as the Sun never gets high in the sky in midwinter. This happens when opposition occurs
near midsummer when the planet is opposite the Sun in the sky and in midsummer the Sun is high, so the planet will be low. The
opposite of course appliesin winter.
The normal cycle for an outer planet is Conjunction – Western Quadrature –
Opposition – Eastern Quadrature - Conjunction. After conjunction, the planet moves away from the Sun as seen from Earth and
becomes visible again. The planet from this point on rises earlier and earlier in the morning sky and
eventually becomes visible in the
evening sky. At Western Quadrature it is at its highest at sunrise and by opposition it is in the same position by midnight. By Eastern
Quadrature, it is past its best and is at its highest at sunset, meaning it is rising in daytime and setting earlier and earlier until a point
when it sets too close to the Sun as seen from Earth and is no longer visible. The duration of this cycle will depend on the planet’s
closeness to the Sun, i.e. Jupiter completes the above cycle in around 13-14 months.