Skywatcher's Guide: April and May 2021


Stars and Constellations

In April, the spring sky is now prominent in the east, along with the familiar Big Dipper.  The Big Dipper is high in the northeast, and the two stars at the end of the bowl can be used to find Polaris, our north star.  Also, the handle can be used to "arc to Arcturus", a bright star in the constellation Boötes. Next, Leo the lion is very high in the east, approaching the middle of the sky now, with the constellation Virgo just below.  The winter constellations are also visible, now in the west.  Taurus the bull is getting lower in the west, near Orion the hunter to the west-southwest.  Canis Major (the big dog), along with the bright star Sirius, is also getting low in the southwest.  Gemini the twins is a little higher in the west, along with Auriga the charioteer in the west-northwest and Canis Minor (the little dog) in the southwest.  The winter Milky Way is now getting lower in the west as well.  Far to the south we can see some constellations making up part of Jason's Argo Navis, which we only get to see briefly since it is so far south.  Finally, there is still a small portion of the fall sky still visible just for the first hour or so of the night.  Cassiopeia the queen is in the north-northwest, and Perseus the hero is in the northwest.

In May, the winter constellations are lower in the west, and Orion, Taurus, and Canis Major are already in the process of setting at the beginning of the night, along with Cassiopeia and Perseus to the northwest.  The spring constellations are higher now, and we can see the rest that weren't up this time last month.  Hercules is below Boötes in the east-northeast.  Some of the summer constellations are also getting ready to come up and will be visible a few hours after sunset.

Interesting Stars Visible in April and May (during observatory hours)

Name / Designation Apparent Magnitude
(lower = brighter)
Sirius -1.44 8.6  
Arcturus -0.05 36.7  
Vega 0.03 25  
Capella 0.08 42  
Rigel 0.18 770  
Procyon 0.4 11  
Betelgeuse 0.45 427  
Aldebaran 0.87 65  
Spica 0.98 262  
Pollux 1.16 38  
Deneb 1.25 3230  
Regulus 1.36 77 means "Little King"
Castor 1.58 52  
Polaris 1.97 431  
Algol 2.09 93 variable star
Denebola 2.14 36.2  
Almak 2.1 / 5.0 & 6.3 355  triple star system w/ 64 yr orbit
Albireo 3.2 / 5.8 & 5.1 390 / 380  possibly a triple star system
Eta Cassiopeiae 3.5 / 7.4 19 480 yr orbit

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Solar System

Mercury will be passing behind the Sun in April, but will be visible in the evening sky during May.

Venus is lost in the glare of the Sun for much of April, but will start to emerge in the evening sky by May.

Mars starts off high in the west after sunset, but gradually gets lower each night.  It will move through Taurus and Gemini.

Jupiter is visible in the morning sky, gradually getting higher and higher.  It is moving from Capricornus into Aquarius.

Saturn stays in Capricornus, slowly separating itself from Jupiter.

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Calendar of Night Sky Events

Date Event
04/04/21 Last Quarter Moon.
04/11/21 New Moon.
04/18/21 Mercury at superior conjunction. — Passing behind Sun.
04/19/21 First Quarter Moon.
04/22/21 Peak of Lyrids meteor shower.
04/22/21 Appulse of Venus and Uranus. — Separated by ?°.
04/24/21 Appulse of Mercury and Uranus. — Separated by ?°.
04/25/21 Appulse of Mercury and Venus. — Separated by ?°.
04/26/21 Full Moon.
04/30/21 Uranus at conjunction. — Passing behind the Sun.
05/03/21 Last Quarter Moon.
05/04/21 Peak of Eta Aquariids meteor shower.
05/11/21 New Moon.
05/16/21 Mercury at greatest eastern elongation. — Visible in the evening sky.
05/19/21 First Quarter Moon.
05/26/21 Full Moon and Total Lunar Eclipse. — Mostly visible from Tucson. Learn more about it here!
05/28/21 Appulse of Mercury and Venus. — Separated by 0.4°.

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Deep Sky

The winter Milky Way is now getting low in the west, but there are still several interesting objects we can see here.  The Pleiades (Seven Sisters, M45) is close to the west-northwest horizon right next to the Hyades (C41), which makes the face of Taurus the bull.  The constellation Auriga is a little bit higher, where we can see M36, M37, and M38, which are visible with binoculars.  The Double Cluster (C14) in Perseus is low in the north-northwest.  Higher in the sky we have the Beehive (Praesepe, M44) in Cancer the crab and Coma Berenices (Bernice's Hair) near the tail of Leo.

We are now beginning to see some globular clusters coming up in the east.  M3 is towards the east, in the constellation Boötes.  Nearby, the famous Hercules Globular (M13) is low to the east-northeast.

For nebulae, we have the spectacular Orion Nebula (M42) now now getting low to the west-southwest. This is the closest star-forming region to our solar system.  We also have some good planetary nebulae, which come from dying stars.  The Eskimo (C39) in Gemini is high in the west, the Owl (M97) in Ursa Major is high in the northeast, and the Ghost of Jupiter (C59) is to the south in the constellation Hydra.

And now the galaxies:  In Ursa Major to the north we have Bode's Galaxy (M81) and the Cigar Galaxy (M82), close enough to be seen together in a low-power telescope.  Nearby in the constellation Canes Venatici we have the Whirlpool (M51), which is a pair of colliding galaxies.  The Pinwheel Galaxy (M101) is also nearby near the handle of the Big Dipper.  Then the Sombrero Galaxy (M104) is in the southeast in the constellation Virgo.

Interesting Deep Sky Objects to Observe during April and May (during observatory hours)

Designation Name Apparent Magnitude Apparent Size Distance
Messier 45 Pleiades 1.6 110' 440 open cluster
Messier 44 Beehive Cluster 3.7 95' 577 open cluster
Messier 42 Orion Nebula 4 85' x 60' 1400-1600 diffuse nebula
Messier 3 (in Canes Venatici) 6.2 18' 34,000 globular cluster
Messier 81 Bode's Galaxy 8.5 21' 1,200,000 spiral galaxy
NGC 3242 Ghost of Jupiter 8.6 25" 1400 planetary nebula
Messier 57 Ring Nebula 8.8 1' 2,300 planetary nebula
Messier 82 Cigar Galaxy 9.5 14' 1,200,000 galaxy

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Frequently Asked Questions

What is dark matter and dark energy?

I'm a bit surprised I haven't answered this question yet.  Dark matter and dark energy are two cutting edge topics in astronomy and cosmology that have yet to be completely figured out.  While the similar terms might be lead them to be easily confused for each other, they are two completely separate ideas.

I'll start with dark matter.  The idea for dark matter originated in the 1880s when British scientist Lord Kelvin noticed that some stars in our Milky Way were moving faster than expected.  He hypothesized that there must be extra mass that could not be accounted for with only the visible stars.  Over the following decades, other astronomers such as Kapteyn, Zwicky, and Babcock expanded on this work.  Then in the 1970s, a team of Vera Rubin, Kent Ford, and Ken Freeman presented the most definitive evidence yet.  By studying rotation curves of multiple spiral galaxies they determined that there must be about six times more mass within these galaxies than what the starlight would indicate.  Other astronomers confirmed this not only in the optical wavelengths, but in radio as well.  That means that it is not simply cold rocks and dust that are not glowing like stars, but that it is some other kind of matter altogether.  Even dust and rocks would emit radio waves, but whatever dark matter is has no signature whatsoever in the electromagnetic spectrum.  It seems to only interact through the gravitational force.  Today, most scientists believe dark matter is some kind of particle that hasn't yet been discovered.  These particles would be flowing through the universe, passing through regular matter undetected, only being manipulated by the force of gravity.  The discovery of such a particle would be an exciting breakthrough in astronomy and particle physics.

Now on to dark energy.  In a sense, dark energy is opposite to dark matter.  While dark matter works with gravity to hold things together, dark energy works against gravity to push things apart.  Einstein first conceptualized dark energy in his theory of general relativity, although he thought it was only a mathematical artefact of his theory rather than a real physical phenomenon.  Edwin Hubble's discovery that the universe is expanding showed that it could in fact be real.  The strongest evidence was discovered in 1998 when astronomers Riess, Perlmutter, and others realized that the universe was not only expanding, but that it was accelerating.  If we only consider gravity, the expansion would decelerate over time, but since that is not what we observe, there must be something continuously pushing outwards in all directions.  Dark energy is not a particle, but is rather a property of space itself.  The more it expands, the more dark energy there is.

The detection of dark matter and dark energy means that the universe we see and interact with is only a tiny fraction of what is actually there.  It turns out that only 5% of the universe is regular matter, with 27% being dark matter, and the remaining 68% dark energy.  We still have a lot to learn about these fascinating concepts.

If you have any questions you'd like me to answer in the next issue of SWG, please let me know.  I'm also happy to take suggestions or comments, and also pictures if you'd like to send them.  Happy viewing!

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  • Cornelius, Geoffrey. The Starlore Handbook: an Essential Guide to the Night Sky. San Francisco, CA: Chronicle, 1997. Print.
  • Ottewell, Guy. Astronomical Calendar 2012. Raynham, Mass: Universal Workshop, 2011. Print.
  • Ottewell, Guy. Astronomical Calendar 2013. Raynham, Mass: Universal Workshop, 2012. Print.
  • Ottewell, Guy. The Astronomical Companion. 2nd ed. Raynham, Mass: Universal Workshop, 2010. Print.
  • Astronomy Magazine. February 2013. Volume 41, No 2.
  • Astronomy Magazine. February 2013. Volume 41, No 3.
  • Sky & Telescope. February 2013. Volume 125, No 2.
  • Sky & Telescope. March 2013. Volume 125, No 3.

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