Archive for the ‘Alignments’ Category

This is the second in a series of articles I’m posting about the transit of Venus on June 6th.  In this one I’ll discuss the 2012 transit and about my plans to observe it from Druridge Bay beach in Northumberland.  I’d hoped to do an article about the history of these events and their impact on astronomy….but I’ve been too busy preparing my telescopes and cameras for this one!

On June 6th the planet Venus, which has adorned the evening sky all year, will slip directly between the Earth and Sun.  For a period of more than six hours Venus will appear silhouetted against the Sun for many observers around the world.  The conditions for seeing a transit are simple: it must be daytime for at least part of the transit.  Here’s a map showing viewing prospects for transit for observers around the world:

At a glance you can see that the UK doesn’t get the best view of transit; we’ll see the end stages just after sunrise on the 6th.  The following picture is a simulation of how the transit may look shortly after sunrise:

The time of sunrise will vary by a few minutes across Northumberland; the further east and north you are the better.  At Druridge Bay sunrise is at 4.28am on the morning of the transit.  The beach is good place to view the transit because the horizon is as low as possible and duration of the transit is maximised.

A clear view of the horizon is essential because any intervening geography may delay sunrise significantly (remember – we only get about 85 minutes of the transit at best from this part of the world!)  NASTRO members are planning to meet at the northern end of Druridge Bay, near Hadston Scaurs, to watch the transit.

Observing the transit will demand the same kind of precautions that are needed for normal solar observation.  Binoculars, telescopes and cameras should be filtered properly to prevent heat damage to the optics (and more importantly – to prevent serious eye damage).  One complicating factor is that at sunrise the light of the Sun may be significantly dimmed by our atmosphere.  This could make it difficult to see the Sun, initially, through filtered instruments.  However, it is still dangerous to stare at the Sun – even if it seems comfortable to do so.  The Sun will brighten rapidly as it climbs higher in the sky.

It’s probably safer to view with experienced observers at your local astronomy club.  In the UK you could take a look at the list of member societies at the Federation of Astronomical Societies to see there are any public observing sessions to attend.

I’ve spent a number of hours in recent weeks getting used to setting up and doing solar astronomy with various telescopes.  I know which eyepieces will give good views of entire Sun or just a small region.  I’ve practised taking pictures with the Nikon D80 on different telescopes with various Barlow lenses and know whether I can focus properly in a given setup and the approximate exposure and other camera settings needed.  All this to minimise wasted time on the morning.  My plan for the transit is as follows.

3.00   Still twilight and reasonably dark.  Get set up: polar align telescopes and other equipment checks.

4.28   Sunrise.  Transit begins!

4.30 – 5.30 Take pictures, watch through the eyepiece.  Appreciate this historic astronomical event unfolding!

5.30   Watch for black drop effect.

5.36   Egress.  Venus begins to leave the solar disk.

5.34 – 5.54   Watch for refraction effects in the atmosphere of Venus.

5.54   Transit ends.

Obviously merely seeing any part of the transit and I’ll consider the day to be a major success.  If I can get pictures of the black drop effect or any images which show the atmosphere of Venus then I’ll be over the moon (or Venus)!

When everything is over, by 6am, we’ll pack up and head to The Trap Inn for a full English breakfast and the probably home.  But not to bed.  A day of image processing and watching pictures from around the world begin to stream in from the internet….

The final part of this series will either be a very short angry tirade about the British weather, or a comprehensive image gallery from our observing session at the beach.  Time will tell.

Advertisements

Nemesis: June 6th 2012

Posted: May 30, 2012 in Alignments

This isn’t an article about the end of the world.  On June 6th Venus will pass in front of the Sun –  as seen from Earth.  Further out in the depths of the Solar System there’s a place which also witness this remarkable event.  It’s an asteroid called Nemesis.  Here’s a rendition of the event by Celestia:

Amazingly, Venus is joined by our own planet Earth.  So really this is a double transit featuring Venus and Earth!

Nemesis is a main belt asteroid with an estimated diameter of 188km and it orbits the Sun at mean distance of 2.75AU.  At the time of the transit Nemesis is close to opposition and about 2.91AU from the Sun.  At that distance Venus subtends an angular diameter of 8 arcseconds, whilst the Earth is a shader nearer (and larger) at 9.3 arcseconds.  These sizes are slightly smaller than Mercury appears during transits seen from Earth.  The Venus transit lasts about 9.5 hours and the Earth transit lasts about 10.5 hours from this vantage point.  The transit of Venus lasts about three hours longer from Nemesis than it does from Earth.  Although the Sun presents a slightly smaller disk at this distance, the angular speed of Venus is smaller – making the its passage across the solar disk last longer.

This is probably a extremely rare event for Nemesis in the same way that double transit would be for us on Earth.  The orbit of Nemesis is tilted about 6 degrees to the plane of the Earth’s orbit.  Nemesis gets to see a double transit because it happens to be at opposition whilst close to a node of its orbit.

I found out about this event because it was noted by Jean Meeus in his book Mathematical Astronomy Morsels III.  Nemesis is not included in Celestia by default; I found the orbit elements from NASA JPL Small Bodies Database and entered them manually.

Weather permitting I’ll enjoy the Venus transit at sunrise from the beach.  And adding to that enjoyment is the knowledge that 270 million miles behind me there’s a big rock watching us pass in front of the Sun!

As I mentioned in a previous post, transits of Venus across the Sun are very rare events.  Mercury can also transit the Sun too – about a dozen times a century on average.  I saw a transit of Mercury on May 7th 2003.  This was the view through my 10 inch telescope;

There’s a sunspot just above the centre of the disk.  Mercury is the tiny speck towards the top.  This picture was one of many taken using a basic digital camera held at the eyepiece of the telescope.  I found the images on an old CD-ROM the other day and had a go at reprocessing them.  The telescope was filtered with a Baader filter paper and the orange colour was added later – an aesthetic choice.

Here’s a zoomed in image showing Mercury a bit more clearly:

The first version I did of this picture actually got published in a journal of the Association of Lunar and Planetary Observers.  Mercury is a small rock about 3,000 miles across and 52 million miles away.  The Sun is about 865,000 miles across and about 42 million miles behind Mercury.  Being so small are far away means that Mercury is a tiny speck against the brilliant Sun – about 1/5th of the apparent size of Venus during a transit.

How do you take images like this?  Well this was my method.

I think I’m under there with a camera!  The next transit of Mercury will be visible from the UK.  It will happen on May 9th 2016.

Venus has been a fixture, or so it seems, in the evening sky since the start of the year.  This superb evening apparition has seen the planet in some beautiful alignments with Jupiter, the Moon and Pleiades.  Only a month ago it was shining brilliantly in the evening sky until well after midnight for UK astronomers.

In recent weeks Venus has been getting lower in the evening sky – setting earlier and earlier – and in a week or so it’ll be very tough to spot in the evening twilight.  This is how the inner planets of the solar system stood yesterday:

As seen from Earth, Venus is almost in the same direction as the Sun.  In the coming days Venus will continue to swing between the Earth and Sun.  I took this picture of Venus last night with my 8 inch Meade LX10 telescope:

Only a very narrow sliver of the planet is visible.  Not surprising, as Venus is now almost between the Earth and Sun so that it’s the far hemisphere of the planet getting illuminated.  You can see from the orbit diagram that Venus is almost at its closest possible distance from us; the picture above was taken when Venus was just over 30 million miles away.  Through a telescope eyepiece it’s now bigger than any other planet can be.  The crescent was fairly easily seen in my 7×50 binoculars!

Venus will continue to draw closer to the Sun and will pass between the Earth and Sun on June 5th/6th.  Astronomers call this type of alignment an inferior conjunction.  As you may already know, the inferior conjunction on that date will be very special: Venus will be seen by many around the world to be silhouetted against the Sun in an event called a transit.

More about that very soon….

Venus and Jupiter have been converging in the early evening sky for the last few months.  Venus has been climbing higher into the evening sky, whilst Jupiter has been drawing closer to the fading twilight in the western sky at dusk.  At present they shine close together in the evening sky at mid-latitudes for more than four hours after sunset.  Here’s a picture I took the other night from near Alnwick, Northumberland:

On March 13th the pair will be at their nearest to each other in the sky – a spectacular sight.  It’s hard to visualise the solar system in 3D when you see a scene like this.  Here’s a picture of the solar system as seen from above; it depicts the positions of the planets on March 13th 2012 when the planets are at their closest together in the sky.

Venus will be 121 million km from Earth and Jupiter is more than six times that distance further away at 842 million km.  Venus appears brighter not only because it is nearer to us, but also because it is enshrouded with a dense atmosphere with very reflective cloud tops.  Also, notice another alignment.  The planet Mars is almost directly on the other side of the Earth to the Sun.  I wrote about that earlier.

As seen from Earth, the planets Venus and Jupiter will be almost perfectly lined up and we observe them to be in the same direction in the night sky.  The alignment isn’t perfect and Venus will not pass directly in front of Jupiter.  At their closest there will still be a gap of sky with an angular size of three degrees.  You could fit three full moons into that gap on the sky!

On March 13th at around 7.30pm the western night sky will look like this:

Venus glides to the north of Jupiter (above).  The magenta lines show orbits that both planets follow around the Sun and they also show why the alignment isn’t perfect.  First, to explain the differing shapes of the orbits that you see above.  Jupiter’s orbit around the Sun is five times wider than the Earth’s orbit and so we are inside it looking out; it wraps completely around the sky so we’re just seeing a small section of it here.  Venus, on the other hand, has its orbit inside the Earth’s orbit.  We can see the entire orbit in the same direction as the Sun.  In the evening sky scene above, the Sun has just dipped below the western horizon and so we see that part of the orbit to the east of the Sun.

So why doesn’t the orbit of Venus look like a line in the sky?  I mean that’s what we might expect if we see a circular orbit from the side!  Although the solar system is approximately disk shaped when viewed from the side….the orbits are not perfectly aligned with each other.  In most cases the orbits are tilted very slightly to each other (just a degree or so) so that most of the time the planets appear to be strung out on a line across the sky called the ecliptic.  But when they get close together then the tiny little differences in the tilt of the orbits becomes more noticeable.  The tilt of the orbit of Venus is particularly obvious because it’s also the nearest planet to us at times.  So in the picture above we’re looking at the circular orbit of Venus – not from edgeways on – but from slightly underneath.

Date of the conjunction

When two planets are close together in the sky the event is often called a conjunction.  Depending on where you look you’ll find the date of the Jupiter-Venus conjunction listed as March 15th despite the planets being closest together in the sky on March 13th.  Why the difference?  Astronomers have different definitions of conjunction! Two planets are in conjunction when they have the same longitude on the celestial sphere.  Using equatorial coordinates, this means they must have the same Right Ascension.

Here’s a map of the sky (thanks to SkyMap Pro) showing the positions of the planets at closest approach at 10.25pm (GMT) on March 13th.

The vertical lines represent Right Ascension.  Although they are at their closest on the 13th the planets are at different Right Ascensions.  Here is the actual conjunction in Right Ascension:

This map shows the positions on March 15th at 10.37am; both planets are on the same Right Ascension line – the conjunction!  They are a little further apart by this time but the difference is tiny.

On a final note, just as the tilt of the moons orbit around the Earth doesn’t stop eclipses happening – just makes them rarer – then so the alignment of the planets can also occasionally be perfect.  On January 3rd 1818 Venus actually transited in front of the planet Jupiter.  Imagine the view through a telescope!

The next event like this will also involve Venus and Jupiter again but won’t happen until November 22nd 2065 and will much more difficult to observe (aside from the practicalities of being alive on that date!)

Dancing Planets

Posted: February 28, 2012 in Alignments, Imaging, Planets

It’s been an interesting week for observing the moon and planets.  In the evening sky Venus and Jupiter have been on show for ages but the gap between them is rapidly closing up.  They were joined by the crescent moon late last week – it skipped past the planets over successive evenings.

On Friday Feb 24th after sunset I captured this picture from Barrowburn Camping Barn in the Coquet Valley among the Cheviot Hills.

It was a fantastic, clear night (mostly) and very dark compared to my usual observing site at Hauxley.  Saturday night was mostly cloudy but there were enough gaps in the cloud to get some more pictures.  The moon had climbed higher into the sky and was sat next to Venus.  The proximity of the two meant that it was relatively easy to spot Venus a couple of hours before sunset! 

Here’s a closer view of the crescent moon with Venus taken through a Skywatcher 80mm refractor with the camera at prime focus.

The moon’s distance was 403,300 km. Venus was 140 million km away (about 350 times further!)  On Sunday and Monday evening it was too cloudy to see the moon as it bypassed Jupiter in the sky but tonight it was clear enough to get some more pictures.  I drove west of Morpeth – just past a little village called Mitford – and set the camera up just some patchy clouds were drifting over.  This is what I saw:

The moon will move away from this scene  but the show will go on without it.  Mercury will join these planets in the sky after sunset and the gap between Jupiter and Venus will continue to close up, reaching a minimum on March 12th/13th.

On the other side of the sky, the planet Mars is nearly at its closest to Earth.  A couple of hours after Mars clears the horizon, it is followed by the planet Saturn. It’s not often we have an evening sky filled with so many of the brightest planets so make the most of it!

On Boxing Day step outside and look towards the southwest about an hour after sunset.  You’ll catch the slender crescent of a not-yet 2 day old moon hanging in the sky next to the evening star; the planet Venus.

Looking southwest on December 26th at 4.30pm. The moon's size haze been exaggerated for clarity.

Telescopes will show Venus to be…not a crescent.  The phase of Venus is currently gibbous; it’s almost on the opposite side to the Sun to us (1.32AU – 198 million km away) and so looks almost fully illuminated. Compare that to the moon which is almost between the Earth and the Sun – so it’s only about 5% illuminated.  The moon is only about 378 thousand km away.  We have no perception of depth on such astronomical scales!

The scene is even better the following evening.

Looking southwest on December 27th at 4.30pm. The moon's size haze been exaggerated for clarity.

You probably know that the Sun, moon and planets all follow an imaginary line called the ecliptic through the sky.  Events like this Venus-Moon conjunction highlight the tiny differences in the orientation of the orbits of the moon and planets.  It’s easy to see that a line connecting Venus and the moon does not point towards the orange glow where the Sun has set.  Look again in a few nights time and they’ll appear to be almost on the same line.

The ecliptic line – and the tiny deviations from it – show up well in the next picture, taken by the moon-orbiting Clementine spacecraft in 1994:

The Plane of the Ecliptic is illustrated in this Clementine star tracker camera image which reveals (from right to left) the Moon lit by Earthshine, the Sun's corona rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury. Image credit: NASA.

The ecliptic is the path traced out by the Sun as the Earth orbits around it.  The Sun appears to crawl through all the familiar zodiac constellations (and another one) during the course of a year.  The moon and planets have orbits which are slightly tilted to the plane of the Earth’s orbit and so they follow the ecliptic only approximately (so they get to visit yet another constellation).