May 2020 Observing Guide (All Times are ST and are based on an observing location of Belfast)
The Sun rises at 05:45 and sets at 21:00 at the start of the month. By month’s end, it rises at 05:00 and sets at 21:45. May sees the beginning of permanent Astronomical twilight at night which will last until August. It also sees the start of Noctilucent Cloud (NLC) season. Look to the North-West for a white/silvery glow 1.5 – 2 hours after sunset and to the North-East a similar amount of time before sunrise. They can sometimes be faint, sometimes bright. Always keep an eye out for Aurorae also.
Mercury is visible in the evening sky in Gemini in the second half of the month, setting at 23:45 at month’s end when it is mag +0.3. Venus and Mercury lie less than 2 degrees apart low in the NW after sunset on the evening of the 22nd. May is the last month of 2020 to see Venus in the evening sky and it is in Taurus, setting at 22:10 by month’s end. It fades from mag -4.4 to mag -3.7 during the month.
Mars (Capricornus – Aquarius), Jupiter (Sagittarius) and Saturn (Capricornus) are all visible in the morning sky. Mars rises at 04:00 at the start of the month and by month’s end it rises at 02:40. It brightens from mag +0.4 to mag +0.0 during the month.
Jupiter rises at 02:55 at the start of the month and by month’s end it rises at 00:55. It brightens from mag -2.2 to mag -2.4 during the month. Saturn rises at 03:05 at the start of the month and by month’s end it rises at 01:10. It brightens from mag +0.6 to mag +0.4 during the month. Uranus is not well placed for observation. Neptune is visible in the morning sky by month’s end in Aquarius when it rises at 02:45 and is mag +7.9.
The full moon is on the 7th (11:45). The last quarter moon is on the 14th (15:03) with the new moon on the 22nd (18:39). The first quarter moon is on the 30th (04:30).
1st pm the 62% waxing gibbous lies NW of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.
2nd pm the 73% waxing gibbous lies NE of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.
5th pm the 97% waxing gibbous lies NE of Spica (Alpha (α) Virginis, mag +1.0) at 23:00.
9th am the 97% waning gibbous lies NE of Antares (Alpha (α) Scorpii, mag +0.9) at 01:00.
12th am the 74% waning gibbous lies SW of Jupiter and Saturn at 04:00.
13th am the 64% waning gibbous lies SE of Jupiter and Saturn at 04:00.
15th am the 45% waning crescent lies S of Mars at 04:00.
24th pm the 5% waxing crescent lies E of Mercury and NE of Venus at 22:00.
28th pm the 37% waxing crescent lies W of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.
29th pm the 48% waxing crescent lies NE of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.
The Eta Aquarid meteor shower does peak on the 5th, but the radiant only rises in the morning twilight shortly before sunrise in Ireland. In addition, a bright waxing gibbous moon is also around, so no good.
C/2017 T2 (PanSTARRS) is currently mag +8 (at time of writing (atow)) and is predicted to remain around a similar brightness until July. It is at perihelion on the 4th and will be circumpolar all month as it moves from Camelopardalis to Ursa Major. It lies to the N of M81 and M82 on the nights of the 23rd/24th and the 24th/25th.
C/2019 Y1 (ATLAS) is currently mag +8 (atow), but will fade during May. It is circumpolar all month, moving from Cepheus to Camelopardalis to Draco to Ursa Major. It lies to the N of Dubhe (Alpha (α) Ursae Majoris, mag +1.8) on the nights of the 19th/20th and 20th/21st. It lies to the N of Merak (Beta (β) Ursae Majoris, mag +2.4) on the nights of the 23rd/24th and 24th/25th. On the 24th/25th, it also lies to the N of M108 and M97 – The Owl Nebula.
C/2020 F8 (SWAN) is predicted to be mag +3 at perihelion on the 27th, it may be visible low in the N after sunset towards the end of the month in Perseus. It will be a tricky one to spot. Apps like Sky Safari/Stellarium or www.heavens-above.com and https://in-the-sky.org/ can be used to get exact location of comets on a nightly basis. Websites like www.aerith.net , www.ast.cam.ac.uk/%7Ejds/ and http://astro.vanbuitenen.nl are excellent information sources for updated brightness estimates of comets.
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.
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.