April 2021 Observing Guide

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

The Sun

At the start of the month, the Sun rises at 07:00 and sets at 20:00. By month's end, it rises at 05:50 and sets at 21:00.

 The Planets

Regular Stuff

Mercury is at superior conjunction on the 19th and becomes visible in the evening sky by month’s end. At the end of the month, it sets at 22:15 in Aries and is mag -1.1.

Venus is not easily visible this month.

Mars is visible in the evening sky, moving from Taurus to Gemini during the month. At the start of the month, it rises during daylight hours and sets at 02:20, by month’s end it sets at 01:50. It fades from mag +1.3 to mag +1.6 during the month.

Jupiter is a morning object in April, moving from Capricornus to Aquarius. At the start of the month, it rises at 05:55 and by month’s end, it rises at 04:10. It brightens from mag -1.9 to mag -2.1 during the month.

Saturn is a morning object in April in Capricornus. At the start of the month, it rises at 05:30 and by month’s end, it rises at 03:40. It brightens from mag +0.8 to mag +0.7 during the month.

Uranus is visible in the evening sky in Aries at the start of the month when it sets at 22:30 and is mag +5.9. It is at superior conjunction on the 30th so not visible at month’s end.

Neptune is not easily visible this month.

The Moon

The last quarter moon is on the 4th (11:02) with the new moon on the 12th (03:31). The first quarter moon is on the 20th (07:59). The full moon is on the 27th (04:32).

Regular Stuff

1st am the 85% waning gibbous lies NW of Antares (Alpha (α) Scorpii, mag +0.9) at 03:00.

2nd am the 75% waning gibbous lies NE of Antares (Alpha (α) Scorpii, mag +0.9) at 03:00.

14th pm the 7% waxing crescent lies S of M45 – The Pleiades at 22:00.

15th pm the 12% waxing crescent lies W of Aldebaran (Alpha (α) Tauri, mag +0.9) and E of M45 – The Pleiades at 22:00.

16th pm the 19% waxing crescent lies NE of Aldebaran (Alpha (α) Tauri, mag +0.9) and SW of Mars at 22:00.

17th pm the 27% waxing crescent lies NE of Mars at 22:00.

22nd pm the 76% waxing gibbous lies NE of Regulus (Alpha (α) Leonis, mag +1.4) at 22:00.

25th pm the 98% waxing gibbous lies N of Spica (Alpha (α) Virginis, mag +1.0) at 22:00.

26th pm the near full moon lies SE of Spica (Alpha (α) Virginis, mag +1.0) at 22:00.

29th am the 95% waning gibbous lies N of Antares (Alpha (α) Scorpii, mag +0.9) at 01:00.

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 Lyrids peak during daylight hours on the 22nd with a ZHR of 18. The radiant is visible as soon as darkness falls on the nights of the 21st/22nd and 22nd/23rd and it gets higher as the night goes on. Unfortunately a bright waxing gibbous moon in Leo will drown out all but the brightest meteors, not a good year.

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

Asteroid (9) Metis is at opposition on the morning of the 5th and is mag +9.5. It can be found in Virgo and is visible as soon as darkness falls on the night of the 4th/5th.

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

Comets

C/2020 R4 (ATLAS) is mag +9 and makes its closest approach to Earth on the 23rd. It starts the month in Aquila before moving into Hercules, Corona Borealis, Bootes before ending the month in Canes Venatici. At the start of the month, it is visible from 03:00, by mid-month from 23:00 and by month’s end, it is visible as soon as darkness falls.

It lies near to Eta (η) Aquilae, mag +3.9 on the morning of the 1st. It lies near to Delta (δ) Aquilae, mag +3.4 on the morning of the 7th. On the evening of the 19th, it lies near to Sarin (Delta (δ) Herculis, mag +3.1). On the evening of the 25th, it lies between Nusakan (Beta (β) Coronae Borealis, mag +3.7) and Delta (δ) Bootis, mag +3.5). Finally on the evening of the 30th, it lies near to the globular cluster - M3.

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. 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.