What’s Up! July 2021

A monthly look at astronomical events in the sky and on Earth 

Compiled by John Rowland

Anyone who has read these articles of mine before will know that I never try to make a silk purse out of a sow’s ear. If something isn’t worth getting excited about, I say so. Hyping things only disappoints. So I will be frank: for the rest of the summer and well into the autumn, apart from the ever-reliable Moon, solar system objects are going to be very disappointing. And July is of course the second lightest month in the year, starting as it does only 10 days after the summer solstice, so deep sky object observing is very limited as well. Consequently, this What’s Up will be a bit downbeat. I’m sorry, but that’s how it is. We must be content to look forward to August and beyond when observing opportunities improve dramatically.

Please note that all 4-figure times given in this article are in BST.

July Objects

The Sun

The Sun remains doggedly inactive. It is due to emerge from solar minimum but as yet there is no sign of it doing so. It is however worth checking spaceweatherlive.com to see whether anything’s going on.

 

You may have managed to see the partial solar eclipse last month. If you did, you may also have been a bit disappointed that the percentage of the Sun covered by the Moon was less that you expected. The popular media – and even some professional astronomy sites that should have known better – were making statements like, “30 – 40 percent of the Sun’s disc will be covered by the Moon”. The problem is that these sites have interpreted what is professionally known as the magnitude to mean the percentage coverage.

 

By definition, the magnitude of an eclipse is the fraction of the angular diameter of the Sun occulted by the Moon at maximum eclipse. For partial or annular eclipses, it will be less than 1.0; for totals it will be equal to or greater than 1.0 because the Moon’s angular diameter is greater than the Sun’s. Magnitudes are often expressed as a percentage as well.

 

Consider the diagram below. The circles are the same diameter and each is centred on the edge of the other. If this represented the Moon (on the right) partially eclipsing the Sun (on the left), we would say that the magnitude was 0.5 or 50%. But it’s perfectly obvious that the dark section does not represent 50% coverage. The Moon may have intruded by 50% of the diameter but it has not obscured 50% of the area.

Fig.1 Double Circles

 

What really matters with a solar eclipse is the fraction of the area obscured by the Moon. This fraction is known as the obscuration. You’ll be interested to know that for equal diameter circles when the magnitude is 50%, the obscuration is only 39.1%. In fact, the obscuration is always less than the magnitude, and when the magnitude is low, the obscuration is even proportionally lower. The table below illustrates the relationship between magnitude and obscuration.

Magnitude % Obscuration %
0 0
10 3.7
20 10.4
30 18.8
35 23.5
40 28.5
50 39.1
60 50.5
70 62.4
80 74.7
90 87.3
100 100

 

The magnitude from York was 35%, so only 23.5% of the Sun’s disc was covered.

 

The Moon

July’s new moon is at 0217 on the 10th. The best evenings to observe the Moon are from Monday 12th (slender crescent) through to Wednesday 21st (late gibbous). However, during that period the Moon doesn’t get much higher than 20° above the horizon due to the angle of the ecliptic. You might want to look over to the WNW horizon on the 12th at about 2200 hrs, when both the Moon and Venus should be spotted about 5° apart. Mars is about half a degree below left of Venus but is 1/180th as bright, so it would be a miracle if you saw it. But maybe. For those wishing to spot the very earliest crescent moon, the best day will be Sunday 11th after the Sun sets at 2131 (See Crescent Moon Watch page for more details).

 

The Planets

Mercury

Mercury is only visible early in the morning (0415 in the ENE) before sunrise.

Venus

Venus is gradually catching up with us following its journey from behind the Sun but due to the low angle of the ecliptic at this time of year, Venus is only about 10° above the western horizon at sunset. This situation will continue right through until the end of the year.

Mars

Tiny Mars is now virtually unobservable. We’re going to have to wait until its next opposition in December 2022 before it presents itself once more.

Jupiter and Saturn

Both these gas giants reach opposition next month (Saturn on the 2nd and Jupiter on the 20th). During July, you’ll need to wait until after midnight to see them low in the SE. Their altitude of less than 15° renders them poor subjects for observing. Best wait until August/September when they climb to 17°(Saturn) and 22°(Jupiter) earlier in the night.

Uranus and Neptune

Both these planets are morning objects and not worth struggling over until later in the year.

 

Deep Sky Objects

July is a challenging month to observe deep sky objects because of the sky brightness, but it is also the month when some famous ones are visible. These are clustered in the direction of the galactic centre and therefore low in the sky. If you don’t have a go at them during the first 2½ weeks of July or August, you’ll miss out until next year. But you need to accept that these objects can only be well seen very late at night, e.g. around 0100 BST, but if you’re keen enough, your efforts will be rewarded.

 

The sky view below shows these objects at 0100 on the 10th.

Fig.2 July DSOs (SkyView.com)

 

Some of the objects listed in the table below can be seen with the naked eye and all are well seen in binoculars or a small telescope.

 

Messier No. Magnitude Description
M11 5.8 The Wild Duck Cluster. One of the richest open clusters (2,900+ stars).
M16 6.0 The Eagle Nebula, cluster and cloud of hot gas. Famous for the Pillars of Creation feature at its centre.
M17 6.0 The Omega Nebula. A cluster of 35 stars within cloud of gas.
M21 5.9 Open cluster of about 40 stars. Impressive in a small telescope.
M22 5.1 Bright globular cluster of about 70,000 stars.
M23 5.5 Loose galactic cluster of about 150 stars
M25 4.6 Compact galactic cluster of about 86 stars of various colours.

 

Noctilucent Clouds

July is a great month for looking out for the eerie and beautiful noctilucent clouds. (From the Latin for night shining.) These are ice crystal clouds, and the highest on Earth, at about 50 miles up. They can only really be seen between midnight and 2 a.m. low above the northern horizon if the sky is otherwise clear.  The image below was taken on 19th June 2019.

Fig.3 Noctilucent Clouds

Noctilucent clouds shot by Nando Harmsen of Fstoppers

 

Man’s Space Activities

 

Spotting the International Space Station

July is a brilliant month for sighting the ISS as it’s visible on nearly every night of the month. On some nights, you can see it three times! The Heavens Above web site gives all the details.  The link is preset to return the data for sightings from York. Use the left and right chevron buttons to change the date range. All times are BST.

 

CHINA’s Tianwen-1 Mars Mission

A few more images have been released, but don’t get too excited. See this updated Sky & Telescope article.

 

Amazon boss Jeff Bezos to take a trip into space

Weather permitting, Jeff Bezos, his brother Mark and a mystery passenger who’s paid $20m at auction for the privilege, will take an 11 minute ride into space on 20th July aboard the New Shepard spacecraft designed and built by Bezos’s Blue Origin company. Some people are petitioning that Jeff be banned from returning to Earth! See this informative Metro article.

 

General Space News

There’s always something of interest going on in space. See the space.com web site for the latest news. And for a detailed timetable of projected launches and other space-related activities, the Wikipedia web site is an excellent place to start as it has links to each project/mission mentioned.

 

And Finally – What’s That?

Fig.4 The-cat-in-the-sky

“The cat in the sky”. Can you say where this is? Be the first to identify it.
Send your answer via the Leave a Reply facility below.

I wish you clear skies and good viewing. Stay safe!

John Rowland

5 thoughts on “What’s Up! July 2021

  1. I think the cat in the sky is in the pillars of creation ; in the eagle nebulae. In Hubble image, it is in first pillar on left hand side , half way down . Freda
    thanks for the newsletter

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