May 2021 Observing Guide

Hi all,

May is the month for Mercury. It has its best evening appearance in 2021 and lies close to M45 – The Pleiades, The Moon and Venus during the month. May should also see the beginning of the Noctilucent Cloud season. Enjoy the below guide.

OBSERVING GUIDE

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

The Sun

At the start of the month, the Sun rises at 05:45 and sets at 21:00. By month's end, it rises at 04:55 and sets at 21:50

The Planets

Conjunctions

3rd & 4th pm Mercury passes to the S of M45 – The Pleiades.

9th pm Venus lies to the SW of M45 – The Pleiades.

28th pm Mercury lies less than a degree to the East of Venus.

Regular Stuff

Mercury has its best evening showing of the year and is at greatest eastern elongation on the 17th. It moves from Aries to Taurus during the month. It sets at 22:20 at the start of the month and by month’s end it sets at 22:45. It fades from mag -1.0 to mag +2.8 during the month.

Venus is visible in the evening sky, moving from Aries to Taurus during the month. At the start of the month, it sets at 21:40 and by month’s end it sets at 23:10. It maintains its brightness at mag -3.8 during the month.

Mars is visible in the evening sky, in Gemini during the month. At the start of the month, it rises during daylight hours and sets at 01:45, by month’s end it sets at 00:50. It fades from mag +1.6 to mag +1.7 during the month.

Jupiter is at western quadrature on the 21st and is visible in the morning sky in Aquarius during the month. At the start of the month, it rises at 04:05 and by month’s end, it rises at 02:15. It brightens from mag -2.1 to mag -2.3 during the month.

Saturn is at western quadrature on the 3rd and is visible in the morning sky in Capricornus during the month. At the start of the month, it rises at 03:35 and by month’s end, it rises at 01:40. It brightens from mag +0.7 to mag +0.6 during the month.

Uranus is not easily visible this month.

Neptune is not easily visible this month.

The Moon

The last quarter moon is on the 3rd (20:50) with the new moon on the 11th (19:59). The first quarter moon is on the 19th (20:12). The full moon is on the 26th (12:14).

Regular Stuff

3rd am the 57% waning gibbous lies SW of Saturn at 04:00.

4th am the 47% waning crescent lies S of Saturn and SW of Jupiter at 04:00.

5th am the 36% waning crescent lies S of Jupiter at 05:00.

13th pm the 4% waxing crescent lies SE of Mercury and NE of Venus at 22:00.

14th pm the 8% waxing crescent lies NE of Mercury and SW of Mars at 23:00.

15th pm the 14% waxing crescent lies SW of Mars at 23:00.

16th pm the 22% waxing crescent lies NE of Mars at 23:00.

19th pm the 51% waxing gibbous lies N of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.

23rd pm the 91% waxing gibbous lies NE of Spica (Alpha (α) Virginis, mag +1.0) at 23:00.

26th pm the just past full moon lies NE of Antares (Alpha (α) Scorpii, mag +0.9) at 23:00.

31st am the 73% waning gibbous lies S of Saturn and SW of Jupiter at 03:00.

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.

The Eta Aquarids peak on the morning of the 6th with a theoretical ZHR of 50. However the radiant only rises in the morning twilight shortly before sunrise on the morning of the 6th from Ireland. This leads to a very short observing window with a much reduced ZHR given the very low radiant.

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

Asteroids

There are no bright asteroids at opposition this month.

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

Comets

There are no bright comets visible this month.

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. “If you want to have a safe gamble, bet on a horse - not a comet”, Dr Fred Whipple.

Deep Sky

On the deep sky front this month, galaxies M81 and M82 can be observed in Ursa Major. In Leo, we have several galaxies on view including The Leo Triplet - M65, M66 and NGC 3628. M95, M96 and M105 can also be observed in Leo. The place to really find galaxies is in Virgo. The Virgo Super Cluster can be found here with numerous galaxies on view. Also in Virgo, M104 - the Sombrero Galaxy can be found. In Coma Berenices, there is M64 - the Black-Eye Galaxy. Also check out the constellation Canes Venatici with the globular cluster - M3 and several galaxies including M51 - the Whirlpool Galaxy and M63 - the Sunflower Galaxy. In Hercules, two globular clusters - M92 and the excellent M13 can be observed and in Lyra - M57 - The Ring Nebula can be observed. Finally there are some excellent open clusters in Cancer - M44 - The Beehive Cluster and M67.

General Notes

Always keep an eye out for Aurorae. The night sky does not get fully dark this month. Between mid-May and the early August, Astronomical twilight is present at night. This is when the sun is between twelve and eighteen degrees below the horizon.

Watch out for NLCs - Noctilucent Clouds during May. 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. 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

Information Sources and Links

Sky at Night Magazine Observing Guide – All Rounder

Stardust Magazine – All Rounder

in-the-sky.org/ – All Rounder

www.nightskyhunter.com/ - All Rounder

Philip's Stargazing 2021 – All Rounder

Collins 2021 Guide to the Night Sky – All Rounder

Night Sky Almanac: A Stargazers Guide to 2021 – All Rounder

Yearbook of Astronomy 2021 – All Rounder

www.heavens-above.com – All Rounder

Sky Safari App – All Rounder

Stellarium App – All Rounder

www.timeanddate.com/astronomy/ - All Rounder

www.irishastronomy.org  - Irish Federation of Astronomy Societies Website and Calendar – All Rounder

irishastro.org.uk/ - Irish Astronomical Association website – All Rounder

www.eaas.co.uk  - Northern Ireland Amateur Astronomy Society – All Rounder

eco.mtk.nao.ac.jp/cgi-bin/koyomi/cande/phenomena_en.cgi – Sun/Planets/Moon Only

International Meteor Organisation - www.imo.net/files/meteor-shower/cal2021.pdf - Meteors Only

britastro.org/computing/charts_asteroid.html – Asteroids Only

www.cobs.si – Comet Observation Database

www.aerith.net – Comets Only

www.ast.cam.ac.uk/%7Ejds/ - Comets Only

astro.vanbuitenen.nl – Comets Only

theskylive.com/ - Comets/Asteroids

messier.seds.org/ - The Messier Catalogue website – Deep Sky Only

www.spaceweather.com – Aurorae Forecasts/Naked Eye Atmospherics

Appendix

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 full moon’s width when viewed from the Earth is 30 arc minutes or ½ a degree. This should 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.

A conjunction is when two objects appear to be close to each other in the sky according to the perspective of the observer.

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 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.
 

Conjunctions

28th pm Mercury lies less than a degree to the East of Venus.

Clouded out here

Finbarr.

And it wasn't much better here Finbarr.

Clear skies,

Darren.