April 2024
The Sky Tonight - April 2024
As May brings us into the second half of Djeran the days continue to draw in and sunset is getting earlier every day. Isn’t …
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December 8
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The casual observer
The night sky in summer is all about looking to the east to see the prime real estate of the night sky: The Pleiades, Taurus, Orion, Canis Major and Canis Minor, stretching across the sky. We are looking to the outer parts of the milky way here, away from its centre, and the patterns of stars are unique and easily distinguished. This also makes for great satellite viewing, as the moving bright lights are easy to pick out from the very recognisable background stars.
A good choice of planets joins the show, with Saturn, Jupiter and Mars easily visible over the course of the evening and Mercury and Venus making an appearance in the west in the early evenings. They are joined by the waxing moon in the first and last week of the month as well.
The Geminids meteor shower occurs on the evening of Dec 14 into Dec 15. This is one of the most spectacular showers to appear in the night sky, and worth staying up for. The best time to see these is after midnight looking to the northeast, though be aware that the third quarter moon will be showing its face as the night goes on. In good viewing conditions you may see dozens of meteors per hour, more than 100 per hour if you are in great conditions.
Image: Radiant of Geminids meteor shower.
The summer solstice occurs on Dec 22 (AWST). This is the point in the Earth’s orbit where the Sun’s apparent southerly motion in the daytime sky comes to a stop, and from here on out it will start to appear to move north again. That’s what solstice means after all: ‘Sol’ – Sun, ‘Sistere’ – Stands still. This is also the longest day of the year in the southern hemisphere, and correspondingly the shortest day in the northern hemisphere.
Earth’s tilted rotational axis points in a fixed direction in space, and on the day of the December solstice the planet is in exactly the place along its orbit to point the south pole in the direction of the sun. Even though it is the longest day of year, this doesn’t mean that it is the earliest sunrise or latest sunset, just the greatest duration between them. Sunrise and sunset will continue to wander around for a few more weeks before noticeably shorter days arrive in January.
Special Occasions
Join scientists worldwide on Dec 25, an important day in the scientist’s calendar. Each year we celebrate the birthday of a person who, by the time of his passing, had irrevocably changed humanity for the better. Natural philosophy was his passion and with deep insight he unified the science of earthly mechanics with the motion of the celestial objects. Neat diagrams and elegant calculations showed that the universe could be understood and that the same forces that dictate your life on the ground also control the motion of the planets. You have probably studied his teaching on the laws of motion and universal gravitation, and are invited to join in wishing a happy birthday to Sir Isaac Newton.
Image: Newton calculates the motion of an object under an inverse square force law, showing the orbit to be an ellipse. Arguably the greatest calculation in the Principia.
ISS sightings from Perth
The International Space Station passes overhead multiple times a day. Most of these passes are too faint to see but a couple of notable sightings are:
| Date, time | Appears | Max Height | Disappears | Magnitude | Duration |
| 11 Dec 8:28 PM | 10° above NNW | 59° | 10° above SE | -3.8 | 6.5 min |
| 12 Dec 04:37 AM | 10° above SW | 86° | 10° above NE | -3.7 | 6.5 min |
Table: Times and dates to spot the ISS from Perth
Source: Heavens above, Spot the Station
Dates of interest
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Waxing moon near Jupiter
December 2
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Full moon near Mars
December 8
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Geminids meteor shower peaks
December 14
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Summer solstice
December 22
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Celebration of birth of Sir Isaac Newton
December 25
Planets to look for
All five naked eye planets make an appearance this month. Saturn continues to be visible in the western sky in the hours after sunset, followed by the significantly brighter Jupiter more to the north.
Mercury is visible above the western horizon immediately after sunset and is joined by Venus in the latter days of the month. You will need to be outside early if you want to catch either of them though.
Image: Dec 25, 8pm. Step outside to take a break from your racist uncle and conspiracy theory aunt and enjoy the parade of the planets instead.
Mars is at its closest distance to Earth on Dec 1 and reaches opposition on December 8, meaning it is exactly opposite the sun in the sky. If you point one hand at the sun and the other at Mars, they will be pointing in opposite directions. This is a long way of saying that Mars is rising as the sun sets. Easily visible in the east after sunset, Mars stands out brighter than the nearby red stars Betelgeuse and Aldebaran. If in doubt, look at the bright red one, that’s Mars.
Constellation of the month
Monoceros the Unicorn
Monoceros is a faint constellation in the outer arm of the galaxy flanked by Orion, Canis Major and Canis Minor. With its brightest star Beta Monocerotis shining with a magnitude of 3.76, this very faint constellation is best viewed as the absence of any notable bright stars between Sirius, Procyon and Betelgeuse.
Though it is artistically represented as the mythical unicorn, there isn’t really any specific mythology associated with this constellation as it was first added to star charts in 1612 by cartographer Petrus Plancius.
In 2002 all eyes were on Monoceros as the previously uncatalogued star – now known as V838 Monocerotis – flared up violently in a huge and unanticipated eruption. Observations initially suggested this eruption was similar to a nova event – an eruption caused by the sudden fusion of material deposited onto a white dwarf star by a companion star – but subsequent observations showed that the eruption continued to increase in brightness, eventually shining a million times brighter than the sun before settling down.
The eruption was well studied, and a series of Hubble images shows the spectacular evolution of the light echo of the event.
Image: Light echo of V838 Monocerotis
Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)
The above image needs to be interpreted with care. Despite looking like an expanding shock wave from an explosion it is actually light from the event reflecting off successive layers of interstellar dust before reaching us.
The cause of the event is still uncertain. Some speculation is that the eruption was a helium flash, a short and furious period of fusion in the core of dying low mass stars. Another strong hypothesis is the merger of an 8 solar mass main sequence star with a 0.3 solar mass companion on a highly eccentric orbit. The high eccentricity of the orbit would mean the merge would stir up the outer atmosphere of the stars as they combined, kicking off a huge and extremely bright envelope of gas. There is more science to be done.
Object for the small telescope
Mars
Now is the best time of year to point a telescope at the red planet. With closest approach on Dec 1 and opposition on Dec 8, Mars will be a great telescope target for the next few months.
Image: Mars, with ample cloud cover visible.
Easily visible in the north-eastern sky in the evenings, Mars has variable weather so can change in appearance daily. Sometimes vague surface features can be discerned, and other times a dust storm will smear everything out.
If you are very lucky you may catch a glimpse of Deimos, Mars’s smaller and outermost moon. Inner moon Phobos is so close to mars it is usually lost in the glare of its parent planet and only visible in larger telescopes. Fun fact: The escape velocity of Deimos is only 5.6 m/s. This means that you could literally jump off the small moon and never fall back.
Image: Deimos
Credit: NASA/JPL-Caltech/UArizona
Goodbye InSight
On Nov 26, 2018, NASAs “Interior Exploration using Seismic Investigations, Geodesy and Heat Transport” mission landed on Mars. Because nobody remembers that name, it is called InSight for short. Like any Mars mission, even landing was itself a great accomplishment and the craft performed a propulsive landing under its own thrusters rather than the more complex Sky Crane method used for Curiosity and Perseverance.
Video: Animation of InSight’s propulsive landing on Mars.
Credit: Lockheed Martin
InSight’s two main objectives were to study quakes on Mars by deploying a seismometer and to measure the temperature of Mars beneath the surface by drilling down and deploying a thermometer. Both studies would provide valuable information on how Mars and other rocky planets form and evolve. Like on Earth, seismic vibrations from quakes reflect off different layers of Mars’s interior. By recording quakes and echos, InSight would provide clues to the layers of Mars down to the core. Complementary data from precise telemetry would assist in constraining the seismometer data as well. Temperature data from the thermometer would provide information on how fast heat is escaping from Mars’s core and how fast the planet is cooling.
Image: Artistic representation of InSight on Mars with the seismometer (front left) and mole (front right) displayed.
Credit: NASA/JPL-Caltech
Unfortunately, the thermometer, affectionately called the mole, was unable to drill down through the Martian soil. The unexpectedly tightly packed soil did not provide enough friction for the drill to leverage off, so every strike of the drill caused the mole to simply bounce in place rather than penetrate down. Technicians tried a variety of methods to assist the mole but ultimately the experiment was declared unsuccessful.
The seismometer was much more successful, however. Mars doesn’t have tectonic plates like Earth does, so quakes are caused by the crust rupturing as the planet slowly shrinks as it cools, and the seismometer has detected more than 700 mars quakes. Data from these measurements allowed scientists to conclude that Mars has a molten core about 1800km across covered by a mantle and a thinner than expected crust which also appears to have several sub layers within it.
Image: On May 4 2022 InSight detected a magnitude 5 quake on Mars, the most powerful quake ever measured on another world.
Credit: NASA/JPL-Caltech/ETH Zurich
InSight landed on the Elysium Planitia, a large flat plain on Mars with few surface features, and close to the equator to maximise exposure for its solar panels. Over time, these solar panels have been covered by dust and the lander can no longer generate enough power to keep all its systems running. Engineers have shut down all of InSight’s other functions in order to prolong the life of the seismometer.
Images: InSight’s first selfie (Dec 2018) vs InSight’s last selfie (April 2022).
Credit: NASA/JPL-Caltech
The solar powered Martian rovers Spirit and Opportunity enjoyed a greatly extended mission because of periodic cleaning events where Martian weather blew away dust from their solar panels. The weather at InSight’s location hasn’t been so fortunate, however. Technicians have tried ingenious methods like pulsing the solar panel deployment motors, essentially shaking the panels like a wet dog shakes off water, without luck. Another method, partially successful, involved using the landers scoop shovel to trickle Martian dirt next to the panel. Passing wind blew some of the dirt into the panels at a sharp angle, knocking off some of the dust in the process and slightly boosting power. But all things come to an end and InSight’s mission will be declared over when it misses two scheduled communication sessions with Martian satellites. This is expected to occur sometime around the end of 2022. A fitting way to go for a robot designed to study the deep dark depths of the red planet: not enough sunlight.
You can send a postcard to InSight and the NASA operating team here: Send a Postcard to InSight – NASA Mars. Go show them some love. Until humans set foot on Mars, robotic explorers are our only way to experience the planet close up, and that’s worth celebrating.
Other Space News
Artemis 1 mission continues to impress, though it did blow the doors off the launch tower.
Not to worry though, the SpaceX Starship 14 engine static fire test also nearly blew up the launchpad.
