Some of my readers have queried me as to why the brighter objects in the night sky have negative magnitude values, while the fainter ones have positive values, when, logically (at least to them), it should be the other way around.
For this seemingly “backward” rating system, we can thank the ancient Greek astronomer Hipparchus, who, in 129 BC, drew up the first recognized star chart. On this chart, he listed the magnitude (from Latin magnitudo or magnus meaning “great”) of the stars he could see in the night sky. Hipparchus listed the brightest stars that he could see with his naked eye as magnitude +1.0 stars, those half as bright as the magnitude +1.0 stars as magnitude 2.0 stars, and so on, until reaching magnitude +6.0, the faintest he could see.
His magnitude scale remained in use for rating the brightness of the stars (and other celestial objects by comparison) for the next 1,400 years. It wasn’t until 1609, when Italian astronomer Galileo (1564-1642) developed his first telescope and observed much fainter stars than those listed on the star charts in use at that time, that the magnitude scale was extended (with ascending positive numbers) to include the fainter stars.
In the mid-1850s, when astronomers discovered that some magnitude +1.0 stars are brighter than others, the scale was again extended outward, this time with ascending negative values to reflect the brighter stars.
The stars Rigel (Orion), Capella (Auriga), Arcturus (Bootes), and Vega (Lyra) were listed at magnitude 0.0, while stars brighter than these were given negative values. Sirius, the brightest star in the night sky, is rated at magnitude -1.43 , while our sun is rated at magnitude -26.7.
Planets and other celestial objects can also be rated on the magnitude scale. Venus, at its brightest, shines at magnitude -4.4, while the full moon beams (on average) at magnitude -12.6.
The faintest stars that the average human, naked-eye can see (under a clear sky from a dark site) is magnitude +6.0, while binoculars can boost that to magnitude +10. In contrast, the Hubble Space Telescope can see stars as faint as magnitude +30.
What does it mean?
A star’s apparent brightness or luminosity refers to the amount of light energy (from thermonuclear fusion within the star’s core) it emits, and how much of that energy passes per second through a square meter of the star’s surface area. Basically, how bright a star appears depends on how much of its light energy per second strikes the area of a light detector (in our case, the human eye). The apparent brightness we see or measure is inversely proportional to the square of our distance from the star, with the apparent brightness diminishing as the distance squares.
Astronomers use the terms “apparent magnitude” and “absolute magnitude” when denoting a star’s brightness. Apparent magnitude is how bright the star appears to an Earth-bound observer, and is directly related to a star’s apparent brightness.
Stellar measurements in the 19th century indicated that magnitude +1.0 stars are approximately 100 times brighter than magnitude +6.0 stars (i.e., it would take 100 magnitude +6.0 stars to provide as much light as a single magnitude +1.0 star). Subsequently, the stellar magnitude scale was modified so that a magnitude difference of five corresponded exactly to a factor of 100 times difference in brightness., while a difference of one magnitude equaled a difference factor of 2.512 in brightness.
This resulting stellar magnitude rating system was based on a logarithmic scale, with whole numbers, and fractions thereof, indicating varying ratios of brightness (e.g., 0 = 1 to 1; 0.2 = 1.2 to 1; 0.5 = 1.6 to 1; 1 = 2.5 to 1; 5 = 100 to 1, etc.). A star’s apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any dimness of the star’s light caused by the interference of interstellar dust along the line of sight of the observer.
When astronomers want to measure how intrinsically bright a star is regardless of its distance from Earth, they measure the star’s absolute magnitude, or its apparent magnitude if all the stars it is being compared to were placed at 10 parsecs distance from Earth. With one parsec equaling 3.26 light-years (a light-year is the distance light travels through the vacuum of space in one year; approximately 10 trillion kilometres), 10 parsecs equals 32.6 light-years, or approximately 100 trillion kms. A star’s absolute magnitude measures its true energy output (its luminosity).
As with the apparent magnitude scale, the absolute magnitude scale is also “backward”, giving less luminous stars ascending positive values, and more luminous stars ascending negative ones. For celestial objects such as comets and asteroids, the absolute magnitude scale (also with positive through negative values) is based on how bright the object would appear to an observer standing on the sun if the object were 1 AU (149,597,871 kms) away.
This week’s sky
Mercury (magnitude -0.8) is visible low (about eight degrees) above the northwest horizon shortly after 9 p.m., before dropping from view shortly after 10 p.m. This bright but small planet (heading towards its greatest eastern elongation from the sun on June 2) achieves an altitude of 18 degrees in the evening sky by May 31. It reaches its half-phase (called dichotomy) on May 29.
Venus (magnitude -4.3) appears only about 13 degrees above the western horizon shortly after 9 p.m., before setting shortly before 11 p.m.
Jupiter (magnitude -2.5) rises in the southeastern sky shortly before 1 a.m., reaching 22 degrees height in the southern sky before fading from view around 5:15 a.m.
Saturn (magnitude +0.48) follows Jupiter into the southeastern dawn sky around 1 a.m., rising to about 23 degrees above the southern horizon before it fades from sight shortly before 5 a.m.
Mars (magnitude +0.16) rises in the southeast around 2:30 a.m., reaching an altitude of about 20 degrees above the horizon before fading from view a few minutes before 5 a.m.
Currently at magnitude +4.5, Comet C/2020 F8 SWAN is now in the constellation of Perseus – the Warrior Prince. This fading comet will be difficult to see, as it reaches an altitude of only about 10 degrees above the northeastern horizon between 4 a.m. and 5 a.m., before the glow of the rising sun overtakes it. With clear skies and an unobstructed view of the northeastern horizon, it might still be seen in binoculars and small scopes.
Until next week, clear skies.
May 29 – Mercury reaches dichotomy
May 30 – First quarter moon
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at firstname.lastname@example.org.
The Strawberry Moon Eclipse May Be Visible Over Metro Vancouver This Week – 604 Now
Metro Vancouver is in for a treat this week, as we’ll be able to see the Strawberry Moon eclipse shine over the city this Friday.
Named after the red summer fruit, this phenomenon is June’s full moon – or otherwise called the Hot Moon or Rose moon.
This particular moon, however, kicks off 2020’s “eclipse season,” and will be visible during the moonrise and moonset.
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You’ll just have to be ready at either 5:30 am or 8 pm, Friday, to see the eclipse over Metro Vancouver.
So, will you be checking it out this week?
Friday, June 5th is also the day of the second George Floyd protest, happening downtown.
For more Vancouver stories, head to our News section.
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How To Watch The Mesmerising Penumbral Lunar Eclipse This Week – Tyla
A penumbral lunar eclipse is taking place this Friday 5th June – and you may be able to catch a glimpse of the mesmerising spectacle if conditions are good.
A penumbral eclipse is more subtle than a total eclipse but just as fascinating, according to the Royal Observatory Greenwich, explaining that the phenomenon occurs “when the Moon travels only through the outer, fainter part of the Earth’s shadow, or ‘penumbra’.”
They add: “This happens when the Earth moves between the Sun and Moon but the three do not form a perfectly straight line.
“The penumbra causes only a slight darkening of the Moon’s surface, with the Moon still exposed to some direct sunlight, so this type of eclipse is easy to miss.”
This process of passing through the Earth’s shadow not only means that the moon’s surface appears darker, but that it may appear to take on a reddish or tea-coloured tinge.
The Strawberry Moon is the nickname given to the full moon in June. It is said that Native Americans and European tribes would give names to the moon because they used it to map out their yearly calendar and times of harvest.
This Friday’s penumbral eclipse will be visible from most of Europe, Asia, Africa, Australia and the East coast of South America.
It’s worth noting that a penumbral eclipse can be more difficult to see with the naked eye – this is because only a portion of the sun’s light is blocked from reaching the moon.
According to NASA, the eclipse starts at 18.46 BST and ends at 22.04 BST. If you want to try to catch from your window, it will be at its clearest at 20.25 BST.
The moon will be 230,000 miles from the Earth – quite a close point in its orbit – which means that it should appear quite big.
This year’s penumbral eclipse will pass close pass to the giant red star, Antares, which is around 12 times the size of our own sun.
Happy gazing, earthlings.
The June 2020 Night Sky – Portugal Resident
Welcome to the June night sky. This is the month of the summer Solstice in the northern hemisphere. It happens on the 20th this year and, after that date, the Sun will appear to move slightly lower each day in the mid-day sky. June 20 is, therefore, the longest day of the year and the first day of summer.
Although June also has the shortest nights of the year, it’s not short on meteor showers with more than a dozen of them visible during the month. This means that on any dark night in June, you will have a better-than-average chance of seeing a shooting star.
On the 21st, there is an annular eclipse of the Sun. These types of eclipses occur because, at that time, the Moon is slightly further away from the Earth than usual and, therefore, does not cover the solar disc fully and the ring of fire effect will be seen. Unfortunately, this event is not visible from Europe. The eclipse track is mainly over the Middle East and central China, with the famous city of Wuhan just missing out on the ring of fire but seeing an 86% eclipse at 4pm local time.
The gas giant planets Jupiter and Saturn are now rising just after midnight over in the south-eastern sky. They are both in the far southern constellation of Sagittarius.
Jupiter is the brightest of the pair, and this year Jupiter can be used to help find Pluto. This close encounter between the largest and the smallest planets in the solar system will happen three times this year and is called a triple conjunction. This is quite rare and the last time that it occurred was 65 years ago.
Pluto is seven times further away from the Sun than Jupiter and much smaller, so it is more than a million times fainter and can only be seen in a large telescope and a dark sky.
The ringed planet Saturn is always a fine sight through any small telescope with its rings and multiple faint Moons visible.
Jupiter has four major moons, and these are quite easy to see with any small telescope. Jupiter’s Moons were discovered by Galileo using a tiny homemade telescope magnifying about 20 times and this was more than 400 years ago.
The Moon is full on the 5th, last quarter on the 13th, new on the 21st and first quarter on June 28, 2020.
Clive Jackson is the Director of the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.
281 322 527 | email@example.com www.torredetavira.com
To see the June Sky Map click on the pdf link below
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