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Archive for the ‘Southern Cross and Orion’ Category

3 May 2012

Since we wrote the last blog, we have joined a Star Hoppers group led by our US mentor Greg. Most of the members are on the east coast of the US. We are the only two, as far as I know, Down Under. The aim of the group is to apply the classic advice of Sherlock Holmes – learn to “observe,” not simply to “see.”

And what should we observe in the first two weeks of May?

 “Good targets for viewing under dark skies are the Beehive (with binoculars or a low power scope), several double stars in Cancer and Leo or the galaxies in Leo.”  

We have to pick 3 – 5 targets on any given night.

The targets for May are fortunately also in constellations in our southern sky.

Constellations of Cancer and Leo

Stellarium image of constellations of Cancer and Leo (click on image to enlarge)

Waiting, waiting

Primed and ready – we waited for the weather to give us a break.

When weather reports of the last few months promised clear skies, they often didn’t mention the evening clouds that creep in, uninvited, and destroy any chance of viewing the stars.  So we’ve invented a new game to amuse ourselves: it’s called “Wither goes the weather?”

On the evening of the 3rd May, around 8 pm, we were pleasantly surprised to find the sky was clear of pesky clouds. When we walked out on to car deck observatory the panorama was breath-taking even with lights on everywhere and moonlight bathing the street. And the air was cool and calm. Perfect for viewing!

Dish of the sky

Stellarium image: Dish of the sky (click on image to enlarge)

Sure our view wasn’t quite as detailed as this, but with all those fairly large objects in view, we were very excited.

Starting from north – Mars was pretending to be one of the stars of Leo, forming a neat triangle with Regulus and Algieba.

Moving past the fat Gibbous Moon, and going east was Saturn, a golden brooch on the skirt of Virgo.

Looking south over Francis Street were the Pointers doing their job of showing us the Southern Cross.

Southern Cross and Pointers

A slide along the Milky Way and we were looking west at Canis Major (Big Dog) lying on his back wearing Sirius like a trophy on his chest. And caught in the moonlight (or should I say despite the moonlight) a hare, Lepus, being chased by the Big Dog. What a beautiful constellation this is! A neat curve of stars.

Just below the Big Dog is his master Orion slipping down sideways behind the trees!

View of western sky

Our photo: View of western sky (Click on image to enlarge

Spoilt for choice

How to choose three targets? Spoilt for choice, weren’t we? So we did what one does at a smorgasbord crammed with too many delectable dishes. We decided for our first night of star hopping to sample every big star and planet we could see and look for doubles and galaxies and clusters before we committed ourselves to three.

 Using our binoculars we star-and-planet hopped. With the Moon between Leo and Virgo, we were lucky to see Regulus and Algieba, but swinging to the Cross we saw the Jewel Box displaying its contents near Beta Crucis. And in the vicinity was the king of globular clusters – Omega Centaurus.

Going down the Milky Way, watered down by the light of the Moon and the lights on in every home in Francis street (people don’t believe in curtains or blinds), we looked for M41 in Canis Major, and found it rather faint. Caught M42 in Orion’s sword just before the Hunter slipped off on his perpetual chase of the Scorpion.

With the sky still clear at 8.30 pm we decided it was time to bring out the scope and look at the planets. Mars was a nice round reddish disk, but Saturn, despite the light of the Moon, beat the god of war in the beauty stakes. Saturn is certainly the pin-up boy of the planets! We could just see its moon, Titan; the other moons were drowned out by the light of the Moon.

Had we been able to see them, this is what we would have seen.

Saturn with moons

We’ll keep that treat for another night.

Turning our sights to the Moon

Having had our fill of stars and planets we turned the scope with the 6mm eyepiece on the Moon. OMG! The angle of the light from the sun lit up craters and mares perfectly, casting shadows to show them in 3D. We have never seen crater Aristarchus with Vallis Schroteri and  crater Herodotus  in such good light. It was the feature of the night.

Some features of the Moon

Another striking feature in this light was the Bay of Rainbows with crater Bianchini clearly visible as were the other four or five craters in the neighbourhood. And then looking toward the edge of the Moon was a rather angular crater with a prominent peak. I think it is Babbage, or perhaps Pythagoras.

We could even see the Rectii range, an unusually straight range in the Sea of Showers.

Blindingly brilliant were Copernicus, Kepler and up at the top Tycho. But another crater that stood out very clearly, so that we could even discern where its rim is broken, was Gassendi on Mare Humorum. With Gassendi A, this crater looks like an engagement ring with a big diamond.

After a month’s starvation of night sky viewing, we feasted on this evening’s gifts of a clear and still night.  An hour and a half of uninterrupted viewing was more than we hoped for, so when the clouds came over at 9.30 pm, we folded our scope without a word of complaint.

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Our friend Joe wrote, “I did notice the other night the Cross was clearly visible, but then I couldn’t locate Orion, which is usually near enough to Orion.”

So where is the Cross and where is Orion?  I was optimistic I’d be able to answer this question on 15 February, a glorious blue, sizzling summer’s day, followed by a clear night. 

For us, living in Sydney, the Southern Cross is always in the sky. From our perspective, it completes a full circle around the South Celestial Pole every 24 hours. We can’t of course see it in the day time, and even at night its ideal position may be at some awkward hour. Being able to spot it instantly depends on the time of night.

 In the summer months it is a question of waiting till the sky is dark enough and so at about 9 pm, I went out on to the street. It was roughly an hour after sunset and the Cross was visible east of south, between 20° and 30° above the horizon.

Southern Cross

Image from Stellarium: Southern Cross (Click on image to enlarge)

So where is it on its circular path around the South Celestial Pole? To answer this I need to understand our position on Earth and our view of the stars.

The Celestial Sphere

I consulted Stellarium and here’s where I found things getting a little complex. We live on a big ball, the Earth. And to mark where we live, geographers have drawn a grid of imaginary lines which they call the parallels of Latitude and the meridians of Longitude. These help us get a fix on where exactly we are on the Earth.

When we look up at the sky we see the sun, moon, planets and stars. To work out where these are, astronomers also draw a grid of imaginary lines. They pretend the sky is a big sphere – a Celestial Sphere around the Earth.

Celestial Sphere

Celestial Sphere (Click on image to enlarge)

The Celestial Sphere has a grid of imaginary lines similar to the ones on Earth. The imaginary lines are projected upward from those of the Earth, so that above the Earth’s equator is the celestial equator; and above Earth’s poles are the celestial poles. All objects in the sky are shown on the Celestial Sphere. We have to pretend the stars are fixed, while the planets, sun and moon move around.

Celestial Sphere

Celestial Sphere (Click on image to enlarge)

http://www.oneminuteastronomer.com/astro-course-day-2/

 I realise the Celestial Sphere is a very practical tool for us when we talk about the position of any celestial object.

Southern Cross on Celestial Sphere

The image below (from Stellarium) shows where the Southern Cross is on the Celestial Sphere. It is straddling the -60° circle of the grid; the Pointers cycle along following the same circle of celestial latitude.

Southern Cross on the Celestial Sphere

Southern Cross on the Celestial Sphere (Click on image to enlarge)

Why minus? I cast my mind back to my school days…  I remember the globe of the earth in the geography room.  I recall how the lines of latitude and longitude are numbered. 

Grid lines

The lines of latitude are circles. At the equator, the circle is 0°. Every ten degrees up a circle is drawn. In the northern hemisphere the 80° circle is near the arctic. The 90° circle is a point – the North Pole.

How latitude is drawn

How latitude is drawn (Click on image to enlarge)

From the equator, the southern hemisphere has the same numbering. To distinguish them we call them north and south latitudes.

Just as on Earth we have north latitude and south latitude, the Celestial Sphere has +(plus) for north and –(minus) for southern celestial latitudes.

There’s another difference. While the heavens above have exactly the same latitude and longitude as the earth below, the naming convention is different.  The celestial latitude is called declination and is measured in degrees.  The celestial longitude is called right ascension, and is measured in hours.

Lines of Declination and Right Ascension on the Celestial Sphere

Lines of Declination and Right Ascension on the Celestial Sphere (Click on image to enlarge)

Declination is like latitude projected on the Celestial Sphere.

Right Ascension is like longitude projected on the Celestial Sphere.

Where we live on our ball, the Earth, determines what we see in the Celestial Sphere. I’m going to take a little journey to see what I can see.

Travelling south

In the unlikely event that I’d be intrepid enough to go to the South Pole during the southern winter, I’m trying to imagine which stars I would see there.

The skies would be dark for most of the 24 hours. I’d see half the sky all the time and nothing would rise or set – all would simply revolve around me. When I look at Orion which straddles the Celestial Equator, I’d see half of the constellation that is south of the equator because it would never set. The half of Orion to the north of the equator would never rise.

Here’s an image from Stellarium which shows what is visible from the South Pole on the 21 June  – our winter solstice in the southern hemisphere. The sky is dark, the sun does not rise.  The sky is stars, stars, stars all 24 hours! Lie back and enjoy the view!

View from the South Pole

View from the South Pole at Summer Solstice (Click on image to enlarge)

 Travelling north

I decide to leave the icy wilderness and start sailing towards the equator. Each night that I move away from the pole a portion of the sky falls out of sight – sort of behind the hill.  More and more stars appear to rise and set, rather than go round and round. When I reach Sydney, which is on approximately 34°S latitude, the area of the sky between the -60° circle and the South Celestial Pole is visible 24 hours a day. Of course I see the stars only at night time.

I see diminishingly less of the southern skies as I travel towards the equator.  So from Darwin I’d see less of the southern skies than I do from Sydney.  The Cross would be much lower on the horizon, and wouldn’t be circumpolar; it would rise and set. 

And when I get to the equator every star on both sides of the Equator – even the North Star which is not really right on the pole, but off by almost a degree – will appear to rise and set. 

It’s quite a journey, but I’m beginning to appreciate what living on a ball, inside a sphere is like. Fascinating!

So, after that diversion, which helped me form a clearer picture of what we see and why, I return to the problem I started out with.

Which way to Orion?

Once I locate the Southern Cross, I travel across the sky towards the north and there Orion is, almost overhead.

Southern Cross and Orion

Image from Stellarium: Southern Cross and Orion (Click on image to enlarge)

 

In February, Orion rises in the east rather early – about 4 pm. So at 9 pm, he is overhead i.e. nearly 60° above the horizon. A big chunk of the Milky Way lies between Orion and the Southern Cross.

Which way to Orion from the Southern Cross?

Which way to Orion from the Southern Cross? (Click on image to enlarge)

If we look at the positions of Orion and the Southern Cross on the Celestial Sphere, we can see how far apart they are. Orion on 0° parallel and the Southern Cross on -60° parallel; roughly 2/3rds of the dome is between them. Here’s a sky map from Stellarium of them with grid.

Southern Cross and Orion on the Celestial Sphere

Southern Cross and Orion on the Celestial Sphere (Click on image to enlarge)

Now here’s a puzzle! If I were at the South Pole in deep summer, would I see Orion?

No, because in deep summer, the sun does not set at the Antarctic Circle – no dark night sky – no stars!

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