April 2020

Planets to look for

Venus continues to hover above the evening twilight low in the northwest – but only for another 6 weeks (until the middle of May.) With not much else to look at out in the west now is the time when we start get questions about “that bright light on the horizon, doing weird things,” and it may seem to jump and dance about in erratic ways the closer it gets to the horizon. This is just due to the Earth’s atmosphere in between you and the planet, which is a much thicker slice to look though rather than looking straight up, and the moving air bending the light coming from Venus around in all sorts of interesting ways. During the 3^rd and 4^th of April it passes over the famous Pleiades cluster of stars.

Jupiter and Saturn start their move back to evening skies this month, rising around midnight at the start of April and progressing to rising at 11pm by the end of the month. This puts them high overhead at dawn. Jupiter is very close to the position of Pluto in the sky on the 12^th, but all this month if you are looking at the largest planet in our solar system you are also looking in the direction of Pluto.

Mars has moved on past the gas giants and spends April passing through the stars of Capricorn. It rises around 12.30am during April.

Mercury is visible in the morning sky for the first half of April, before it dives into the dawn twilight during the latter half of the month. The Moon doesn’t join it until the 22^nd, which by then it has sunk into the twilight glow.

Constellation of the month

Cancer the Crab

Cancer is said to represent a crab who was annoying Hercules while he tried to slay the nine-headed Hydra. Hercules crushed him underfoot in the fight and Hera, always liking someone who had a go at Hercules, placed him in the sky. This seems a rather weak reason for Cancer’s place amongst the stars but we may never know why as Cancer’s origins are far older than the Greeks’ naming of it, a common occurrence in star lore. (Cancer is not named after a disease, but the Latin name for Crab. Early doctors thought that cancers looked crab-like and that is how they became known as such.)

The Tropic of Cancer, running along latitude 23.5⁰ north, is a relic from two thousand years ago, when the Sun reached its most northerly point in the sky in this constellation. If we were to name the Tropic line today it would be the Tropic of Gemini, due to the effects of precession.

Cancer doesn’t have many bright stars in it, so you find it either by locating Gemini to the west and Leo to the east and looking in between them, or looking for the Beehive Cluster that sits in the middle of it. The Beehive Cluster (Messier 44) is still marked as Praesepe on most star charts. Praesepe means the Manger, and the two bright stars either side of it are known as the Northern Donkey (Asellus Borealis) and the Southern Donkey (Asellus Australis).

Rho Cancri, better known by its Flamsteed number 55 Cancri, but now also known as Copernicus, as renamed in the first round of renaming of stars with exoplanets, is a double star that has five exoplanets circling around it. The last planet found orbits within what is sometimes called the “Goldilocks Zone”, the zone where is the planet is the right distance from its star (that is, not too close and not too far) for liquid water to be found. But since the planet itself seems to be a gas giant, we will have to search for water on any moons found circling it.

Cancer occupies an area of relatively faint stars between Gemini and Leo.

The Beehive cluster is a large open cluster of stars found in the middle of the constellation of Cancer the Crab.  An open cluster is a group of stars that all formed out of the same cloud of dust and gas. In a dark sky you can see this cluster without the aid of binoculars or telescope, but in the city you may need some help.  Locate Gemini to the west and then Leo to the east and roughly halfway in between is the Beehive cluster.

The Beehive stands out well at low power (it actually looks best in binoculars). You should be able to spot a square house with a triangular roof, much like a stick-figure drawing of a house, which is the hive and the hovering bees are the surrounding stars. This cluster lies about 520 light years away.

First Weather Satellite Launched, 1960

One of the most heavily visited websites in Australia belongs to the Bureau of Meteorology (BoM). We tend to take for granted the amazing work they do sampling, modelling and interpreting what our chaotic atmosphere is doing and will do in the future. We can now know when to prepare for heatwaves or cyclones and know when stormy cold fronts are likely to arrive to within the hour. And they couldn’t do it to the accuracy they do without satellites watching from all around the globe. Imagine being a meteorologist up in the tropics in the early 20^th century with only a barometer and random reports from shipping to give you any hints as to whether there was a cyclone off the coast! Now we can even predict where the cyclone will form even before they spin themselves up – but forecasting where they will end up is still a bit of a specialised skill.

The first weather watching satellite was launched on the 1^st of April in 1960. Called Tiros-1, it was an experiment to see how useful a satellite might be in gaining data to add to the usual temperature, pressure and wind direction readings from the ground. Scientists also wanted to see what else they could learn about the Earth from up high. Armed with a TV camera and two magnetic tape-reels to record the images before transmission back down to Earth, it was a great success, and the technology has steadily improved with each decade.

L: Working on the Tiros-1 satellite before launch. No clean rooms back then! R: The first image sent back from Tiros-1.

Today the BoM receives satellite images every 10 min from the Japanese satellite Himawari-8, which watches the entire hemisphere where we are, and data from some Chinese satellites over the Indian Ocean to see what is coming from the west over the Indian Ocean. Satellites watch the whole globe now, monitoring things like sea ice at both poles, the amount of volcanic dust in the atmosphere for aviation routes, ocean currents, snow cover, dust storms and other significant events. Most of the basic data is available free on the internet.

Earth isn’t the only planet that has an atmosphere and experiences weather. All the major planets expect Mercury have weather of some sort, the moons Titan at Saturn and Triton at Neptune both have atmospheres and even Pluto has a thin atmosphere of nitrogen. So if you like space and you like weather it is possible to combine the two subjects.

Weather in the solar system! Clock-wise from top-left: Lightning storms on Jupiter; the thick acid-laden clouds of Venus; the moon Triton has a thin atmosphere created by the geysers on its Neptune-facing side; winds up to 2000 km/hr blow white clouds into long streaks around Neptune; the thin nitrogen atmosphere of Pluto as seen from behind by New Horizons in 2015; Titan, Saturn’s largest moon, clouds and rain of methane; the hexagonal-shaped cyclone at the pole of Saturn; once a decade or so huge global dust storms are suspended in the thin air around Mars, hiding the surface features from view for several months.