Surface Brightness vs Magnitude

A few years ago I wanted to have a look at the Triangulum Galaxy M33 from my yard in Coquitlam. There’s a significant amount of light pollution – I’d estimate the sky to be Bortle class 7 to 8. Despite that, I am able to see bright Messier objects like the Globular Cluster M13, the Ring Nebula M57, and the Bode’s Galaxy M81. M33 was more challenging. I tried several different nights but had no success in spotting it. Confused and frustrated, I checked the magnitude in Stellarium as I knew magnitude is a measure of an object’s brightness. Stellarium showed M33’s magnitude as 5.7 – quite bright and definitely brighter than the other objects I could see: M13 at 5.8, M57 at 8.8, and M81 at 6.94 (remember that brighter objects have a lower magnitude). So why couldn’t I pick out M33?

The Triangulum Galaxy M33. Image Credit: Keesscherer / CC BY-SA (

I have since learned that magnitude works well for stars but it is not a good indicator of how easy or hard it is to see deep sky objects that cover an extended area – aka extended objects. Magnitude assumes all the light is concentrated in a single point source like a star but M33 covers an area of approximately 65 x 40 arc minutes and its light is spread over this entire area.

Simulated Surface Brightness with 2 Squares
Simulated average vs total brightness – on the left, the light is spread over a faint 2×2 square while on the right, the light is spread over a 1×1 square. The total brightness for both squares is the same but the average brightness is much less for the square on the left because its area is 4 times larger.

A better measure is “surface brightness”: the average brightness over the extended area. Surface brightness is often measured in magnitudes per square arcsecond. M33 has a surface brightness of 23 mag/arcsec^2. If you divide the area of M33 into a bunch of little 1 arcsecond squares then on average each square will be as bright as a 23rd magnitude star. As with magnitudes, a lower surface brightness indicates a brighter object. An object with a surface brightness greater than 22 is generally considered to be faint.

Surface brightness is also a good way to measure sky-glow and light pollution. An urban/city sky might have a surface brightness of 17 mag/arcsec^2 while a pristine dark sky might have a surface brightness of 22 mag/arcsec^2. The Canadian company Unihedron makes relatively inexpensive Sky Quality Meters (SQM) used by many amateur astronomers to measure sky-glow. I used Unihedron’s SQM-L and measured the sky-glow at my Coquitlam location to be 18.5 mag/arcsec^2 on a moonless night.

An object can be difficult to detect when its surface brightness is close to or fainter than the sky-glow. Tony Flanders reports barely seeing objects down to a surface brightness of 21 in his skies where the sky-glow is 18 mag/arcsec^2 and he speculates that an object is not detectable if it is more than 3 magnitudes fainter than the sky-glow. With this in mind, it is not surprising that M33 is hard to detect from my Coquitlam location as the surface brightness of M33 is almost 4.5 magnitudes fainter than the sky-glow.

There are other factors involved in whether or not an object is visible. The RASC Oberver’s Handbook section “Magnification and Contrast in Deep-sky Objects” has a good explanation of some of these including the following.

  • Surface brightness is not uniform. For example spiral galaxies often have a bright core area that is brighter and easier to see than the arms. Flanders and others suggests that peak surface brightness is an even better indicator of whether or not an extended object is difficult to observe.
  • Larger objects are easier to detect so increasing the magnification on faint objects can help.
  • Objects like nebula are easier to see when using band filters that increase contrast by darkening the sky-glow more than they darken the light from the nebula.
  • Surface brightness may not be a good measure for objects like globular and open clusters that resolve into individual stars.
  • The surface brightness of an object is subjective as it depends on the size of the object. How far do the faint arms of a spiral galaxy really extend? 
Enable the “Additional Information” in Stellarium’s Configuration to display the selected object’s surface brightness.

Several tools can help you find the surface brightness for observing targets:

  •  The surface brightness of an object can be displayed in Stellarium if the “Additional information” option in the “Configuration” window is enabled.
  • The “Magnitude and Contrast Calculator” is a spreadsheet, included as a supplement to the RASC Observer’s handbook, that calculates the surface brightness and predicts whether an object is visible from telescope, filter, and sky parameters.
  • Tony Flanders has compiled a list of Messier objects that includes the surface brightness and peak surface brightness (for some Messier objects). The table below is an excerpt from his list.
M57Planetary NebulaLyr8.817.817.81.4×1.0
M45Open ClusterTau1.219.8100
M27Planetary NebulaVul7.318.420.18.0×5.7
M76Planetary NebulaPer10.118.620.42.7×1.8
M24Star CloudSgr3.120.595×35
M1Supernova RemnantTau8.420.56×4
M13Globular ClusterHer5.816.920.617

Perseid Meteors from Comet Swift-Tuttle

Comet Neowise is receding from us but the Earth is about to plow through the debris field left behind by the comet 109P/Swift-Tuttle. We’ll see this as the Perseid Meteor shower where dust to pea-size bits of comet material impact the atmosphere and burn up in a trail of glowing ionized gas.

Perseid Meteor Shower from 2015, Image Credit: mLu.fotos from Germany / CC BY (

This annual meteor shower occurs over several days, every year, in mid-August. This year, the peak rate is predicted to occur on the night of Tuesday, August 11th at 11:00 pm PDT but Perseid meteors will be active for several days surrounding the peak.

Comet Swift-Tuttle is a periodic comet that makes a return trip close to the Sun and Earth every 133 years. It comes a little inside Earth’s orbit at its perihelion before heading back out past Pluto. It has a long history of observations with probable sightings as early as 322 BC and 69 BC. In 188, Chinese records suggest it reached naked-eye 0.1  magnitude. Its last close approach was in 1992 when it was visible with binoculars. The next return trip is not until 2126 when it could again be a bright naked-eye comet at 0.7 magnitude.

Radiant for the Perseid Meteor Shower in Stellarium

Perseid meteors can appear anywhere in the sky but they all seem to originate from a fixed point, called the radiant,  near the star Eta Persei in the constellation Perseus – the Hero.  From a dark site, you may be able to see as many as 110 meteors per hour. Note that the Moon rises just after midnight, at 12:05 AM, on August 12th and the moonlight will wash out faint meteors during early morning observing. No special equipment is needed to see the meteors – just lie back in a lawn chair or on the ground and look up and perhaps count how many you see.

The MacMillan Space Centre is hosting a Perseid Meteor Shower celebration on Wednesday, August 12th. The in-person star party is sold-out but you can still register to watch a live stream (by donation) – visit the Space Centre’s event info page to register and for more information.

Three Missions to Mars

Three missions to Mars are getting ready to launch in the next few weeks. Why are all these missions launching at the same time?

Atlas Rocket at Kennedy Space Center
NASA Mars 2020 – Atlas Rocket at Kennedy Space Center – first launch opportunity begins at
4:50 a.m. PDT on July 30. Image credit: NASA Mars 2020.
Positions of the Sun, Earth, and Mars at Opposition.
Positions of the Sun, Earth, and Mars at Opposition. Image credit: Association of Lunar and Planetary Observers.

They are all timed to match the upcoming Mars opposition on October 13th, 2020. When Mars is at opposition, Mars and the Sun are on opposite sides of the Earth. At opposition, Mars is also at its closest point to the Earth, making it more fuel-efficient for a spacecraft to reach the red planet.

Missions to Mars are often designed to follow a Hohmann transfer orbit to move a spacecraft from a lower circular orbit to a higher circular orbit using the least amount of fuel. A Hohmann transfer orbit involves an initial burn to push the spacecraft into an elliptical orbit, coasting most of the way to the higher orbit, then a second burn to put it into a circular orbit. To meet up with Mars, the transfer orbit also needs to be timed so that the spacecraft reaches the higher orbit at the same time that Mars is there – this timing requirement leads to launch windows that occur a few months before each opposition of Mars. The Mars Insight spacecraft ( launched in May 2018, with the trajectory below, was timed to match up with Mars’ last opposition in July 2018.

Animation of InSight's trajectory from 5 May 2018 to 26 November 2018
Animation of InSight’s trajectory from May 5th, 2018 to Nov 26th, 2018. Image credit: HORIZONS System, JPL, NASA

Oppositions of Mars occur approximately 26 months apart so missing a launch window means a delay of over two years.

United Arab Emirates – Emirates Mars Mission

Launch Window: July 14 to Aug 12

This science mission from the United Arab Emirates (UAE) — the first by an Arab-Islamic country — will blast-off from Tanegashima Space Center, Japan. The “Hope Probe” seeks to provide a better understanding of the Martian atmosphere and its layers, including the loss of hydrogen and oxygen gases into space over a Martian year.

China National Space Administration (CNSA)’s – Tianwen-1

Launch window: July 23 to Aug 15

The Tianwen-1 mission (known as Huoxing-1, HX-1 during development) will be China’s first Mars orbiter, lander and rover. It will deploy an orbiter around Mars and land a rover on the surface on April 23, 2021. Its stated objectives are to search for evidence of both current and past life and to assess the planet’s environment.

NASA Mars 2020

Launch Window: July 30 to Aug 15

The Mars 2020 mission addresses key astrobiology questions about the potential for life on Mars. The Perseverance rover includes a drill that can collect core samples of the most promising rocks and soils and set them aside in a “cache” on the surface of Mars – ready to be returned to Earth in a future mission.

Delayed: European Space Agency – ExoMars

A fourth mission, the ESA’s ExoMars mission, was originally scheduled to launch in 2020. However, the COVID-19 pandemic impacted the timelines for testing vital parachutes & electronics and forced a delay. But since oppositions of Mars happen every 26 months, the launch has been postponed by over two years to 2022.

How to Find Comet Neowise

The once-in-a-decade Comet Neowise is a spectacular sight in the evening skies from Vancouver.

Comet Neowise from Vancouver on July 15th 2020 at 10:30 PM PDT

Here are a few tips for finding the comet.

  • Check that the skies are at least partly clear – Vancouver’s Clear Sky Clock: provides a forecast of the cloud cover.
  • Try to find a dark site, away from city lights if possible, with a clear view to the north-west.
  • Look to the north-west, above the north-shore mountains.
  • The comet is visible a little after sundown starting around 10:00 pm.
  • It is low on the horizon – at approximately 15° or the width of your hand held out at arms length.
  • It is below the Big Dipper. Use the two bottom stars in the bucket of the Big Dipper as a pointer to the end of the tail.
  • The comet appears as a faint fuzzy streak to the naked eye.
  • The view is spectacular in binoculars. The tail is at least 5 degrees long so it fills most binocular’s field of view.
  • The comet moves down closer to the horizon as the it gets later at night.

The comet should be a nice sight for the next few weeks and there are some clear evening skies in the forecast so take the opportunity to have a look. Gary Boyle wrote an article in the Georgia Strait with more information & tips. The article includes the image below showing the progress of the comet during July.

Chart showing the progress of Comet NEOWISE across the night sky until the end of July 2020.STELLARIUM.ORG. Image Credit: Gary Boyle, Georgia Strait

The comet is a great target for photography so bring your camera and get some advice on taking your own photos in the article “A Beginner’s Guide To Photographing Comet Neowise “. Vancouver is Awesome has a great collection of photos from around Vancouver.

The comet will dim in absolute brightness as the month progresses because it is moving further away from the Sun. But it is moving higher in the sky and makes its closest approach on July 23rd so the views and contrast might still improve for another week.

Comet Neowise from Vancouver at its Closest Approach to Earth, July 23th 2020, at 10:30 PM PDT

NOVA Newsletter for July/Aug 2020

Our NOVA Newsletter for July-Aug 2020 is available as a pdf file. An archive of older issues can be found on our Newsletter page. The contents include:

Barry Shanko 1960-2020 by Chris Gainor

Is the Sun Getting More Active? by J. Karl Miller

President’s Message by Gordon Farrell

The RASC Virtual GA and GA Lite 2020 by RASC Vancouver GA Committee

Memories of Barry by Scott McGillivray

GA and GA Lite Recordings on YouTube

Recordings of the RASC Virtual General Assembly and GA Lite sessions are being uploaded to the following playlists in the RASC Vancouver Channel on YouTube Logo.

RASC Virtual GA 2020 Playlist


RASC GA Lite 2020 Playlist


Videos from “GA Lite – Part 1” are there now, others will be uploaded by the end of June or early July. Subscribe to the RASC Vancouver channel to get notifications when new videos are uploaded.

Happy Summer Solstice – The Earliest Since 1896

By Milan B

The (Northern) Summer Solstice, today at 2:44 PM PDT, marks the earliest beginning of summer since 1896.

Chart of Solstice dates in Vancouver classified by "type" (the remainder after dividing the year by 4). Years evenly divisible by 4 (type 0 - such as 2016, 2020, 2024) can have early solstices.
Solstice dates in Vancouver classified by “type” (the remainder after dividing the year by 4). Years evenly divisible by 4 (type 0 – such as 2016, 2020, 2024) can have early solstices.

The chart above shows how the quadrennial correction (which the Julian Calendar is solely based on) works well short term, but over a longer period of time (130 years or so), it produces a significant error (one full day). To correct this, the Gregorian Calendar introduces another correction which is to omit leap days in some of the years that centuries begin with (or end, some would say). Years like 1700, 1800, 1900, 2100, 2200, 2300 etc. are chosen for this correction. So, we are in a deep stretch without this centennial correction and the seasons’ start dates will be falling even earlier until late 21st century.

Comet C/2017 T2 near Galaxy M109

Our observing director, Robert Conrad, wrote this post on facebook about comet C/2017 T2 (PANSTARRS).

“Don’t miss the opportunity on the night of June 15th to see comet C/2017 T2 (PANSTARRS) in the same FOV as the bright galaxy Messier 109 and the bright star Gamma Ursa Majoris. View it around 11:00pm when it’s dark and still relatively high in the sky. Good luck! “

The good news is that the comet is still close to M109 on the night of Tuesday June 16th, 2020 and the skies are forecasted to be somewhat clear in Vancouver. It is easy to find in the North-Western skies because of its proximity to Phecda – a bright star in the bowl of the Big Dipper. Recent observations have the comet at magnitude 8.5 so it is visible in small telescopes.

Galaxy M109 next to the Bowl of the Big Dipper at 11:00 pm PDT on June 16th, 2020 from Vancouver
Comet C/2017 T2 (PANSTARRS) is within 1° of Galaxy M109

GA Lite Astronomy Webinars

Join us for our “GA Lite” series of astronomy webinars, a great follow-up to the RASC Virtual General Assembly. The series is scheduled over the two weekends June 13&14  and June 20&21, with more details at


The webinars are free, open to the public, and family friendly.

What else are you going to do with this Vancouver weather forecast?