The night sky in July 2018

posted 29 Jun 2018, 13:50 by Pete Collins   [ updated 30 Jun 2018, 04:58 ]
by Anne Holt

Sunrise     1st:   04.44        31st:  05.22

Sunset      1st:   21.40        31st:  21.06

Astronomical Darkness    1st:  none     31st:  00.50  to  01.40

The Earth is at its furthest from the Sun (aphelion) on 6th at 17.46

New Moon:   13th  at  03.47

Full Moon:     27th at  21.20.

July's full Moon is known as the Buck Moon because it's when new antlers start to grow.  Other names are Thunder Moon, Wort Moon or Hay Moon (Old English).

Lunar Perigee (closest to Earth)  13th

Lunar Apogee (furthest from Earth)  27th

Highlights

At last we have some real highlights to look forward to. The nights are now beginning to get longer, though not yet by much - sunset is less than a minute earlier each night in early July, rising to a couple of minutes by month end. On 31st we have a return of astronomical darkness, when the Sun will be more than 18 degrees below the horizon for a whole 50 minutes.  The new Moon on 13th is a supermoon, of course it can't be seen but the thin crescent just before and after this should appear larger than usual. There is still a chance of seeing noctilucent clouds shortly before sunrise or after sunset, especially in the first part of the month - see June notes for more details about these.  And, at the end of the month, we might see some fireballs.

The real highlights are Mars at opposition on 27th, towards the end of July it will be at its biggest and brightest for 15 years, and a total lunar eclipse on the same evening (details at the end of this post).  Unfortunately these will be very low in the sky but should be visible from a site with an unobstructed, unpolluted SE horizon - and a clear sky!

Constellations

The Summer Triangle (made up of Vega in Lyra, Deneb in Cygnus and Altair in Aquila) is now quite high in the southern half of the sky. Cygnus, with its Northern Cross asterism, and Lyra are particularly prominent.

At the start of the month Pegasus, followed by Andromeda, is rising in the early hours.

As always during the summer months, it isn't the best time to see the zodiac constellations or planets as the ecliptic never gets very high in the sky.  However, if you do happen to visit a dark sky site over the next few months you should be rewarded with good views of the Milky Way high overhead running through Cygnus and down to Sagittarius just above the southern horizon.

Planets

Mercury:  in Cancer,  mag -0.1. Might be spotted very low in the west soon after sunset at the start of the month.  On 1st it sets at 23.00, about 80 minutes after the Sun, but is only 3 degrees above the horizon at dusk so isn't an easy target.  It reaches greatest eastern elongation on 12th, when it sets at 22.33 and is separated by 26.4 degrees from the Sun. Because of the decreasing angle of the ecliptic to the horizon it is even lower in the sky - on the horizon as it begins to get dark.  It will also have faded to mag 0.4. On 14th it is about 2 degrees South of the 4% Moon soon after sunset.  It moves into Leo on 15th, then can't be seen for the rest of the month.  On 31st it sets about the same time as the Sun and will have faded to mag 2.6. 

Venus:  in Leo, mag -4.1. Still very bright but much lower in the evening sky.  On 1st it sets at 23.40 and is 12 degrees above the horizon at around 22.00 as the sky begins to darken.  Unlike Mercury it can easily be seen while the sky is still bright.  On 9th, when it sets a couple of hours after the Sun, it is close to Regulus (alpha Leonis).   On 15th the 10% Moon passes just over 4 degrees North of the planet.  By 31st it sets at 22.25, 80 minutes after sunset and is only 7 degrees above the horizon at dusk, but is slightly brighter at mag -4.2.

Mars:  in Capricorn,  mag -2.2. Mars is the star of the show in the second part of the month.  On 1st it rises at 23.35 and culminates (reaches its highest point in the sky) 13 degrees above the southern horizon at 03.33 - 80 minutes before sunrise.  On this night it will be about 5 degrees south of the 91% lit Moon, soon after midnight.  During the month the planet brightens considerably as it approaches perihelion (the closest point in its orbit to the Sun). The last week in July it reaches mag -2.8,  brighter than Jupiter at its best.  It will also appear much larger when seen through a telescope, apparent diameter 21 arcseconds on 1st, increasing to 24.3 arcseconds by 31st.  This is only slightly smaller than the maximum possible. At its furthest from us the diameter is only 3.5 arcseconds.  The big difference in size, compared to the outer gas giants, is because we are so much closer to Mars.  At its closest it is only about 35 million miles, at its most distant 250 million miles - more than 7 times further.  By comparison, Jupiter at approx 365 million miles and 601 million miles is just over 1.6 times further at its most distant, so the difference in brightness and size is much less marked.

 Mars reaches opposition on 27th, when it rises at 21.53 and culminates at 01.22.  This is known as a perihelic opposition because it occurs when the planet is only a few days from perihelion.  However, because it's apparent motion is currently retrograde (east to west against the background stars) and because the angle of the ecliptic is decreasing, it will be slightly lower in the sky - only 10 degrees - than in early July. It reaches perihelion on 31st, when it will be at its closest to Earth for 15 years -  and the 2003 opposition was the closest since Sept 24th 57,617BC.  Please don't ask whether this is according to the Julian or Gregorian calendar,  I haven't  a clue!  The next time it will be as close is August 28th 2287. It is best seen after 23.20 when it reaches an altitude of 7 degrees.  It culminates at 01.02, 10 degrees above the southern horizon.

A word of warning:  If the old chestnut appears on the internet about Mars being as large as the Moon in the sky, don't believe a word of it.  The Moon will be 75 times larger, Mars will still appear as a point of light to the naked eye - albeit a very bright orange-red point of light.

Jupiter:  in Libra.  mag -2.3. An early evening object now, best seen in the first part of the month.  On 1st it culminates at 21.14, just before sunset and sets at 01.58.  Best seen after 22.00, when it will be 20 degrees above the SW horizon as the sky darkens.  On 10th it resumes direct, or prograde, motion appearing to move eastwards across the sky.  By 14th it will have faded slightly to mag -2.2 and will be outshone by Mars, now at -2.5. On 31st it is down to mag -2.1 and sets at 23.57. As the sky darkens it will be 15 degrees above the SW horizon.

Saturn: in Sagittarius, mag 0.00. An evening object, still very low in the sky.  On 1st it culminates at 00.56, 13 degrees above the southern horizon, and sets around sunrise.  On 24th it is 2 degrees (or maybe 4 degrees depending on who you believe) SE of the 92% lit Moon at around midnight. By month end it will have faded slightly to mag 0.2, reaching its highest point at 22.45, before the sky is fully dark, and setting at 02.37.

Uranus:  in Aries*, mag 5.9. On 1st it rises at 01.22 but only reaches 1 degree above the horizon by dawn.  May be seen in the later part of the month, on 20th it will be 22 degrees above the eastern horizon at 02.40.  On 31st it rises at 23.21 and reaches 32 degrees in the SE before the sky brightens.  Unlikely to be visible to the naked eye - decent binoculars or a small scope are needed.  A larger scope will show the blue/green disc. * Apologies: last month I said that Uranus was in Pisces, despite it having moved into Aries at the end of April (habit, didn't even look - it was in Pisces for longer than I've been doing these notes.)  Did anyone notice, I wonder?

Neptune:  in Aquarius, mag 7.9. On 1st it rises at 00.13 and reaches 11 degrees above the horizon by dawn.  On 4th the 68% lit Moon passes 3 degrees south of the planet.  On 18th it reaches 25 degrees in the south by 02.35.  On 31st it is 4 degrees north of the 89% Moon, rising at 22.11 and reaching 29 degrees as the sky brightens.  Should be visible as a bright blue disc in a reasonable sized amateur scope.


Dwarf planets and asteroids

Still not a good time for spotting or imaging these faint objects because they are mainly very low in the sky - which, of course isn't completely dark for almost the whole of July.

Pluto: in Sagittarius, mag 14.8. Very low in the sky throughout the month, never getting higher than 14 degrees above the horizon.  On 1st it culminates at 01.59.  Reaches opposition on 12th, 2 days before the 3rd anniversary of New Horizons' historic flyby.  On this day it culminates at 01.15 and at 23.34 0n 31st.

Haumea: in Bootes, mag 7.4 and Makemake, in Coma Berenices, mag 17.1 are possible targets for keen astrophotographers, especially in the first part of the month.  On 1st Haumea reaches an altitude of 31 degrees, and Makemake 27 degrees.

Asteroid Vesta: in Ophiuchus, mag 5.6. Fading during July.  In the first few days of the month it is a naked eye object in theory, though very unlikely in practice, even from a dark sky site, as it is very low in the sky. On 1st it culminates at 00.15, only 15 degrees above the southern horizon.  On 31st it sets at 01.47 and will have faded to mag 6.8.

NOTE: one astro mag says that on 12th it will appear only 0.3 degrees from the Moon.  I couldn't find mention of this anywhere else so I checked the positions.  On this day Vesta is in Ophiuchus and the Moon is passing through Orion and doesn't rise until after Vesta has set.  More like 180 degrees apart.

Ceres and Eris not visible this month.

Recommended sites for more details on planetary positions:

https://in-the-sky.org/ephemeris.php

https://theskylive.com

www.nakedeyeplanets.com

Meteor Showers

 One middling and a couple of minor showers, this month.

Delta Aquarids, active July 12th to August 23rd, peak on the night of 29/30th. ZHR 16, though probably significantly fewer from our northern latitude.  Only a few days past full Moon, so that won't help either.. Medium paced meteors,  parent comet 96P/Macholz. .

Alpha Capricornids,  active July 3rd to August 15th, a fairly long peak around 30th, ZHR  5.  These are slow moving meteors, the shower usually includes fireballs. Parent comet 169P NEAT.  Again, the Moon will interfere and the low radiant means that the shower is better seen from further south.

These two showers both have radiants on the ecliptic so could be confused with meteors from the Antihelion Source, which is active in late July (ZHR 2 -3).  These are faster paced than meteors from either of the showers so should be distinguishable.

There could be a few visible meteors from the Gamma Draconids. An outburst of 100 was observed 1n 2016 and we could have a similar number this year.  But don't get too excited -  this year the peak is predicted around midday on 28th, so will only be detectable using radio or radar equipment.

Sporadic activity is usually good in July and we may see some early Perseids in the second half of the month.

Comets

Still no bright, or even fairly bright, comets.  The best one Pan/STARRS(2016M1) at mag 9 is now only visible from the southern hemisphere. From our latitude we have 21P Giacobini-Zinner, currently at mag 13 or 14 (guess what? As usual, sources fail to agree) in Cygnus, moving towards Cassiopeia and expected to brighten considerably over the next couple of months, maybe even becoming a naked eye object from late August.

see www.cometwatch.co.uk/current-observable-comets/  for more detail.

And finally, leaving the best until last ...

On July 27th we have a total lunar eclipse or, to be precise, half an eclipse, as the Moon will be fully in shadow as it rises at 21.07,  47 minutes after the start of totality. At 21.21, the mid point of the eclipse, the Moon will be only 1.4 degrees above the horizon.  By the time totality ends, at 22.13, it will have reached an altitude of 6.8 degrees and be almost directly SE.  The partial phase finishes at 23.19, with the Moon at an altitude of 12.4 degrees.

The very low altitude during totality means that in order to see this event you will need to be somewhere with a a clear SE horizon with no light, or other, pollution to spoil the view.  The Moon will appear very small in the sky,  known as a Micromoon as it is close to apogee (furthest point from Earth). This does mean that it will be moving more slowly than when it is closer to us.  Also at this time the Earth is only 3 weeks past aphelion, when the umbral shadow is at its widest and longest of the year.  These, combined with the fact that the Moon will pass through the shadow very close to the centre - the widest point - mean that this eclipse will be the longest of the 21st century. With 1 hr 43 minutes of totality it is only 4 minutes short of the maximum possible.  The eclipse of July 16th 2000 (which counts as the 20th century) was slightly longer, at 1 hr 46.4 minutes.

 As you may have realised, this eclipse is on the same night that Mars reaches opposition.  However, Mars rises at 21.53, only 20 minutes before the end of totality, so it is very unlikely that both will be visible at the same time.   The partial phase of the eclipse ends at 23.19 so it might be possible to see Mars and a partial eclipse together - given the right conditions.

And, on that night there's a chance of some fireballs, though they will be in the opposite part of the sky so everyone will probably miss them.

The bad news is that, statistically, we only have a 13% chance of clear skies.

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