Skyguide - with Neill McKeown

Neill McKeown

OBSERVING GUIDE
(Please note all times are ST and are based on an observing location of Belfast and covers the month of September)

The Sun

At the start of the month, the Sun rises at 06:30 and sets at 20:15. By month's end, it rises at 07:25 and sets at 19:00.

The Planets

Conjunctions

1st and 2nd am Venus and M44

5th am Mars and Regulus (Alpha (α) Leonis, mag +1.4)

10th am Mercury and Regulus (Alpha (α) Leonis, mag +1.4)

16th and 17th am Mercury and Mars

20th am Venus and Regulus (Alpha (α) Leonis, mag +1.4)

Mercury is at greatest western elongation on the 12th and is visible mid-month before sunrise. On the morning of the 12th, it rises at 05:10 and is mag -0.2.

Venus is a morning object in September. It rises at 03:30 at the start of the month, by month’s end it rises at 05:00. It maintains it brightness at mag -3.8 during the month.

Mars is a morning object this month. It rises at around 05:20 and maintains its brightness at mag -1.8 during the month.

Jupiter is not visible this month.

Saturn is at eastern quadrature on the 14th and is visible this month in Ophiuchus. During the month, it is visible as soon as darkness falls and sets at 21:50 by month’s end. It fades from mag +0.4 to mag +0.5 during the month.

Uranus is visible in the evening sky this month in Pisces. At the start of the month, it rises at 21:25, by month’s end it rises at 19:30. It maintains its brightness at mag +5.7 and lies near to Torcularis Septentrionalis (Omicron (ο) Piscium, mag +4.3) during the month.

Neptune is at opposition on the 5th and is visible in the evening sky this month in Aquarius. At the start of the month, it rises at 20:20 and by month’s end it rises during daylight hours. It maintains its brightness at mag +7.8 and lies near to Lambda (λ) Aquarii, mag +3.7 during the month.

The Moon

The full moon is on the 6th (08:03). The last quarter moon is on the 13th (07:25) with the new moon on the 20th (06:30). The first quarter moon is on the 28th (03:54).

5th pm the waxing gibbous lies W of Neptune at 22:00.

9th pm the waning gibbous lies SE of Uranus at 22:00.

12th am the waning gibbous lies NW of Aldebaran (Alpha (α) Tauri, mag +0.9) and S of M45 – The Pleiades at 00:00.

13th am the waning gibbous lies SE of Aldebaran (Alpha (α) Tauri, mag +0.9) at 00:00.

18th am the waning crescent lies SE of Venus and Regulus (Alpha (α) Leonis, mag +1.4) and NW of Mercury and Mars at 06:00.

19th am the waning crescent lies SE of Mercury and Mars at 06:00.

21st pm the waxing crescent lies W of Jupiter at 20:00.

22nd pm the waxing crescent lies NE of Jupiter at 20:00.

25th pm the waxing crescent lies NW of Antares (Alpha (α) Scorpii, mag +1.0) at 20:00.

26th pm the waxing crescent lies NW of Saturn at 21:00.

Meteors

The best time to observe meteor showers is when the moon is below the horizon; otherwise its bright glare limits the number you will see especially the fainter ones. Below is a guide to this month's showers.

There are no bright meteor showers this month.

There may be additional minor showers this month, details of which can be found in the below Information Sources and Links Section.

Asteroids

Asteroid (89) Julia is at opposition on the evening of the 6th in Pegasus. It is mag +9.0 and visible from 20:00 on the evening of the 6th.

Finder charts and further information about other fainter asteroids can be found in the below Information Sources and Links Section.

Comets

Comet C/2017 O1 (ASAS-SN) peaks around mag +7 in October and is currently mag +10. It will start the month in Taurus, before moving into Perseus by month’s end. It passes to the W of Lambda (λ) Tauri, mag +3.4 around the 7th. It then passes to the E of M45 around the 17th and to the E of Menkhib (Zeta (ζ) Persei, mag +2.8) around the 25th.

Finder charts and further information about the above and other fainter comets can be found in the below Information Sources and Links Section. Any of the above estimates are based on current information at the time of writing the guide and can be wrong - “Comets are like cats; they have tails, and they do precisely what they want”, David H Levy.

Deep Sky

On the deep sky front this month, galaxies M81 and M82 can be observed in Ursa Major. In Hercules, two globular clusters - M92 and the excellent M13 can be observed and in Lyra - M57 - The Ring Nebula can be observed. In Vulpecula - M27 - The Dumbbell Nebula can be found. In Andromeda, M31 - The Andromeda galaxy can be observed along with its satellite galaxies M32 and M110. In Perseus, there is the open cluster M34 and the excellent Double Cluster - NGC 869 and 884. In Triangulum, there is the galaxy M33. Finally Auriga is reappearing with its three open clusters M36, M37 and M38 as is Taurus with the excellent Pleiades - M45 and the Hyades.

General Notes

Always keep an eye out for Aurorae. The autumn equinox is on the 22nd which sees the end of summer and the beginning of autumn. This is the day where the length of day and night is the same and after this the night will take over cumulating with the shortest day of the year on the winter solstice in December. Other interesting naked eye phenomena to look out for include the Zodiacal Light and the Gegenschein. Both are caused by sunlight reflecting off dust particles which are present in the solar system.

The Zodiacal Light can be seen in the West after evening twilight has disappeared or in the East before the morning twilight. The best time of year to see the phenomenon is late-Feb to early-April in the evening sky and September/October in the morning sky - it's then that the ecliptic, along which the cone of the zodiacal light lies, is steepest in our skies. The Gegenschein can be seen in the area of the sky opposite the sun. To view either, you must get yourself to a very dark site to cut out the light pollution. When trying to observe either of these phenomena, it is best to do so when the moon is below the horizon. A new appendix has been added explaining some of the more technical terms used in the guide.

Clear Skies

Neill McKeown

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 zenith. 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 Perseus. 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 which 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. There are 60 arcminutes in a degree.

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 General catalogue whose objects have the prefix NGC. There are links for websites to both catalogues in the section above.

Perihelion is the point in the orbit of a planet, asteroid or comet where it is at the nearest point in its orbit to the sun. It is the opposite of Aphelion, which is when the object is at the farthest point in its orbit from the sun. For the earth, the comparative terms used are perigee and apogee and for the moon, pericynthion and apocynthion are sometimes used.

The Planets

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 planets 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 it's at its furthest point west from the sun as seen from Earth and visible in the morning sky in the East before sunrise. 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 the autumn and before sunrise in the spring. This means that any of the planets will be difficult to 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 applies in 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.