Bright skies need bright objects!
- Credit: Torbay Astronomical Society
Stargazing with Torbay Astronomical Society's John Stapleton
I have said both last month and this that the sky has not been dark enough for the observation of faint objects.
This is because there are three twilight times:
- civil twilight, when the geometric centre of the Sun reaches up to 6⁰ below the visible horizon; this is when street lights come on and cars should use headlights
- nautical twilight, when the centre of the Sun reaches 6⁰-12⁰ below the sea horizon
- astronomical twilight, when the centre of the Sun reaches 12⁰-18⁰ below the horizon and the last rays of sunlight still cause a glow on the horizon.
When the Sun lies lower than 18⁰ below the horizon night begins.
From the end of May through to mid-July the Sun never gets lower than 18⁰ below the horizon so there is no true night.
Fortunately, there are bright objects for us to look at. But what do we mean by bright?
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The first attempt to quantify brightness was made in the second century BC by the Greek astronomer Hipparchus and by the second century AD, another Greek, Ptolemy, had devised a system of six classes of brightness.
Ptolemy’s system was based on how 'big' a star appeared to the naked eye – bright stars look 'bigger' than fainter stars, but this is an optical illusion.
It was later shown that there was a difference of 100 magnitudes (a word coined by Ptolemy) between the first and sixth classes in Ptolemy’s system.
In 1856, British astronomer, Norman Pogson suggested a logarithmic scale which meant that each order of magnitude was 2½ times brighter than the one before and this is the scale use today.
Pogson chose the bright star, Vega - easily visible overhead in July and one point of the Summer Triangle - as his point of reference and gave it a magnitude of 0 - this is also the maximum magnitude of the planet Saturn.
This means that any object brighter than Vega - and there are many - would have a negative value as its magnitude while any object fainter than Vega would have a positive value.
An object of magnitude 2 is 2½ times brighter than an object of magnitude 3 and so on throughout the scale.
Relative to Vega and Saturn most objects are fainter: the planet Uranus has a maximum magnitude of 5 while Neptune only ever reaches 8.
The dwarf planet Pluto never gets brighter than magnitude 14 which is also the limit of most amateur telescopes in the range of 8-10 inches in diameter.
Most interesting faint objects are between magnitudes 6 and 10, or lower. The limit of 7x50 binoculars is magnitude 10 and the limit of naked eye observation is magnitude 6 - even in rural dark-sky sites it is only extended to magnitude 7.
The Hubble Space Telescope, sitting outside the Earth’s atmosphere can resolve down to magnitude 32.
There are also a variety of objects - and stars - that are brighter than Vega and these are objects worth viewing at this time of year.
The star, Sirius, has a magnitude of -1 and is the brightest star visible in the Northern hemisphere.
The planet Mercury has a maximum magnitude of -2 while Mars and Jupiter both have maximum values of -3.
Venus is significantly brighter with a maximum magnification of -5.
I have given the maximum values for the planets as their brightness changes with their distance from the Earth as they orbit the Sun whilst the stars are always the same distance from the Earth and so have a definite value.
The International Space Station has a maximum value of -6 as it passes overhead while Iridium Flare satellites can reach magnitude -9.
The brightest object we ever see in the night sky, the full Moon, has a maximum magnitude of -13.
However, the brightest object in the sky at any time is the Sun which has a maximum magnitude of -27 and this is why it is dangerous to look directly at it with any kind of optical instrument.
We are fortunate this year to have five of the planets visible in either the evening or morning sky.
In order of brightness they are Venus, Jupiter, Saturn, Mercury and Mars.
Mars is last because it is receding from the Earth and is far from its maximum magnitude.
The first three of these are quite easy to spot.
The star chart
The sky will look like the chart on July 7 at 9pm and again on July 22 at 8pm. And four minutes earlier on each successive night e.g. 8.56 on July 8.
To use the chart hold it above your head while facing south so that you can look directly from the chart to the sky.
Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.
Sun: The Sun is never very far below the horizon at night until the latter half of July, so it never gets astronomically dark, and this is not the best time for observation of faint objects.
Mercury and Venus: Both of these inner planets are closer to the Sun than the Earth so are never very far from the Sun (especially Mercury).
Mercury rises about an hour before sunrise at the beginning of July and is seen against the background stars of Taurus in the east.
Venus is an evening object throughout July and can be seen against the background stars of Cancer.
The planet sets in the west 90 minutes after sunset at the beginning of the month and is still visible more than an hour after sunset at the end of the month.
Venus is easily the brightest object in the night sky apart from the Moon and is often mistaken for a UFO because of this.
Mars: The red planet is still visible but is getting fainter as it moves away from the Earth. It may be seen against the background constellation of Cancer low in the WNW sky.
The best time to spot Mars this month will be on July 13 when it lies just half a degree (the width of the full Moon) from brilliant Venus.
Mars will appear as a faint reddish orange dot close to Venus.
Jupiter: The largest planet rises in the East about an hour and a quarter before midnight on June 1 and another hour earlier by the end of the month.
Jupiter reaches a greater altitude above the horizon this year; about 27 degrees (a little more than the span between your thumb and little finger at arm’s length) so astronomers are hoping for many opportunities to observe the giant planet and its four largest moons, known as the Galilean moons, because they were discovered by Galileo the very first time he looked at Jupiter with his small telescope.
These moons can be seen with a good pair of binoculars or a small (bird-spotting) telescope.
DSLR images of the planet will also pick up the moons.
Watch out for small black dots seen against the surface of Jupiter itself, these are either the silhouette or the shadow of the moons as they pass between us and the giant planet.
This will happen several times between July 17 and 29.
Saturn: The ringed planet can also be found rising in the east about two hours before midnight on June 1 and another hour earlier by July 30.
Saturn also reaches a greater altitude than last year and the rings are beginning to open out, from our point of view.
Saturn’s largest moon, Titan, can be seen to the right of the planet with a good pair of binoculars or a small telescope.
Uranus and Neptune: Uranus rises in the east an hour before midnight by the end of the month and is seen against the background stars of Aries.
Neptune also enters our skies rising in the east about 9.30 and seen against the background stars of Aquarius.
Both of these planets are too low in the sky and too faint for easy observation.
Meteor shower: The Delta Aquarid meteor shower peaks on July 29 but a nearly full Moon and low altitude means that conditions are not favourable to see many meteors however it might still be worth a look if the sky is clear.
Comet: There are no bright comets expected this month.
The Last Quarter Moon occurs on July 1 with New Moon on July 9. First Quarter follows on July 17 with full Moon at the end of the month on July 24.
Noctilucent clouds: It may seem anachronistic for astronomers to be interested in clouds, but these rare apparitions are best seen at this time of year.
Noctilucent clouds are so high (over 80 km) in the atmosphere that they are able to reflect sunlight even when the Sun is well below the horizon.
The tiny ice crystals that make up the clouds reflect bluish-white light that stands out against the gloaming sky.
They are typically seen one-and-a-half to two hours after sunset low in the north west or before sunrise low in the north east.
Data supplied by Simon Harding, Observations Secretary Torbay Astronomical Society
The next meeting of the Torbay Astronomical Society, complying with the current Covid regulations will be online via Zoom.
For details, contact the secretary TAS on firstname.lastname@example.org
Visitors and prospective members especially welcome.