by Steve Owens | Jan 1, 2011
Originally posted on Dark Sky Diary by Steve Owens www.twitter.com/darkskyman
At 1900 GMT on 3 January 2011 the Earth will be at perihelion, its closest approach to the Sun this year.
If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.
The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis.
Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km). The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent?
The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical. This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.
Earth's tilted axis
The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers.
The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (3 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter. it has to happen some time and it’s coincidence that it happens to occur within two weeks of midwinter / midsummer.
To take this explanation even further, we can calculate how much variation in incident sunlight (called the flux) there would be in two scenarios:
1. an imaginary scenario where the seasonal varioations in temperature are due to the tilt of the Earth’s axis but where the Earth goes round the Sun in a perfectly circular orbit
and
2. an imaginary scenario where the Earth’s axis isn’t tilted, but where it’s orbit is elliptical in the same degree as ours actually is.
1. The Sun appears at its highest point in our sky each day at noon. The highest it ever gets is at noon on midsummer. The lowest noontime altitude occurs at noon on midwinter.
In Scotland the Sun is around 55 degrees above the horizon at noon on midsummer, and only 10 degrees above it at noon on midwinter.
The amount of energy from the Sun radiant on a fixed area is proportional to the sine of the altitude, so the ratio of the solar energy radiant on a square metre of Glasgow between midsummer and midwinter is
sin(55) / sin(10) = 1.84
So here in Scotland we get 84% more energy from the Sun in summer than we do in winter, due to the tilt of the Earth’s axis.
2. If the Earth’s axis was not tilted, then we would only experience temperature differences from the Sun depending on how far or near we are from it. In this case, the amount of energy from the Sun radian of a fixed area is proportional to the square of the distance from the Sun, so the ration of the solar energy radiant on a square metre of Glasgow between perihelion and aphelion is
(1.017/0.983)^2 = 1.07
So we get 7% more energy from the Sun at perihelion than we do at aphelion., due to the differing distances to the Sun.
From this you can see that, while the distance to the Sun has some effect on how much heat we receive, it is a very small effect compared to that produced by our axial tilt.
by VirtualAstro | Dec 23, 2010
A round-the-world wave to the humans aboard the International Space Station by their fellow humans on the Earth – choreographed by a grassroots Twitter campaign (@ISSwave).
24-31 DECEMBER, 2010
A celebration of human solidarity during the holiday season
For one week beginning Friday, 24 December, humans around the world will show their solidarity with their fellow humans in space (and on Earth) by waving at the International Space Station (ISS) as she passes overhead at 17,500 mph (28,000 kmph).
Participants, recruited through Twitter, are encouraged to share their waves — either alone or as part of an ISSwave tweetup (a physical gathering of twitterers, or tweeps) — by tweeting their zip/postal code and the hashtag “#ISSwave” along with photos and videos of their waves, thoughts, holiday wishes for the astronauts and cosmonauts, etc. Participants’ waves will be registered in real-time at www.isswave.org.
Astronauts and cosmonauts aboard the International Space Station may even film themselves waving back at ISSwave participants. At least two astronauts, including Ron Garan, have voiced their support for ISSwave in emails and tweets.
The idea for the wave emerged through a serendipitous twitter exchange among Twitter acquaintances and regular ISS watchers Lucy Rogers (@DrLucyRogers), Richard P. Grant (@rpg7twit) and Karen James (@kejames). They discovered that watching ISS passes is even more exciting when done together with other humans, whether they are standing right next to you or watching from afar. To know that you are not the only one looking up in awe at this spectacle of human ingenuity and cooperation speeding across the night sky creates a special connection between us.
“The first time I watched an ISS pass I was surprised by how much it affected me,” said Karen James. “‘We made that’, I thought, ‘there are humans up there!’ All of my worries just seemed so tiny in the face of this symbol of human achievement and cooperation. I want to share that experience with other humans and also show my support to the ones living and working aboard the station.”
‘“I’d always wave up at the ISS if I saw it pass overhead,” says Lucy Rogers. “Someone laughed and said the astronauts wouldn’t see me.” So she asked on Twitter if anyone else waved – a lot of people did – and the communal ISS waving began. “When Karen moved to the USA she saw the ISS at a different time to us in Europe – which prompted the idea of a round-the-world wave,” she says.
We see the ISS because it is lit by the Sun. Sunlight reflects off it’s solar panels in the same way it glints off windows here on Earth. As the ISS travels round the world, the reflection can be seen in a broad sweep across the Earth. Due to the angles involved between the Sun, ISS and our location on Earth, sometimes we see bright, high passes and sometimes we can’t see it at all. During the week 24th – 31st December, most places on the Earth should get a good view of it at some point.
The three formed the Twitter account @ISSwave to coordinate, promote and provide updates on the event. Their hope is that seasoned and novice ISS watchers alike will experience the startlingly emotional experience of an ISS pass, amplified by solidarity with thousands of others watching around the world.
Additionally, the team hopes the buzz around ISSwave will persuade those who have never watched an ISS pass to participate, marking an increase in awareness about the International Space Station and the existence of a community of space enthusiasts on Twitter (“spacetweeps”).
The wave also celebrates the 10th anniversary of continuous human presence in space (ISS10years) on 2 November 2010 and the 50th anniversary of Yuri Gagarin’s flight into space — the first human spaceflight — on April 12th 2011 (www.YuriGagarin50.org).
ISS Wave Info:
- The International Space Station has been orbiting the Earth over 15 times a day for more than ten years.
- Although it is about 390 km (~240 miles) high, we can still see it from the Earth, thanks to the Sun reflecting off the solar arrays. The solar array wingspan is 240 feet (73 meters). This is longer than that of a Boeing 777 model at 212 feet (65 meters).
- Currently on the ISS are Oleg Skripochka, Alexander Kaleri, Dmitry Kndratyev, Paolo Nespoli, Catherine Coleman and Scott Kelly (Commander).
- Photos of the ISS passing overhead are available at http://www.isswave.org/ISSWave/Media_Photos.html
- There are various ways you can work out when it will be possible to see it from where you are, including Heavens Above, Twisst, NASA, ESA and Orbiting Frog.
- As of 19 December, @ISSwave had over 600 followers from across all continents.
- Dr Karen James (@kejames) is Director of Science for The HMS Beagle Trust, a UK charity aiming to rebuild the famous ship that carried Charles Darwin around the word on his seminal voyage of discovery. Through the Beagle Project she collaborates with NASA Astronaut Michael Barratt a long-duration spaceflight veteran and member of the crew of the upcoming STS-133 mission to the ISS aboard Space Shuttle Discovery. She is a former postdoctoral researcher at the Natural History Museum in London and has recently repatriated to the United States. For more information visit http://kejames.com/.
- Dr Lucy Rogers (@DrLucyRogers) a Chartered Mechanical Engineer and Fellow of the Royal Astronomical Society, aims to infiltrate the public’s consciousness by writing scientific stuff in plain English. She has published a book about space flight, “It’s ONLY Rocket Science”, which doesn’t contain any equations. She lives on the Isle of Wight where she can see the Milky Way from her back garden. For more information visit http://lucyrogers.com.
- Dr Richard P. Grant (@rpg7twit) is a biological scientist turned writer, editor and poet. He currently lives and works in London, and has a habit of taking on far too many projects. For more information visit http://rg-d.com/rpg, or his blog at Occam’s Typewriter.
PRESS CONTACT:
For more information or to arrange an interview:
UK: Dr Lucy Rogers
Twitter: @DrLucyRogers
Skype: dr.lucy.rogers
Phone: +44 1983 731 759
Email: [email protected]
USA: Dr Karen James
Twitter: @kejames
Skype: karenejames
Phone: +1 207 669 2663
Email: [email protected]
by VirtualAstro | Dec 15, 2010
This full frame image was taken on the 14th of December from Cemaes Bay on
Anglesey looking East by Kev Lewis
Canon 5D MkII and Canon 24mm f1.4L lens shooting 20 second exposures at
f2.8 iso 1600
http://www.photosbykev.com
by Astroguyz David Dickinsen | Dec 10, 2010
Originally posted on the 10th December 2010 by AstroGuys
http://astroguyz.com/2010/12/10/astro-event-don%E2%80%99t-miss-the-geminids/
Looking Northeast at about 10 PM. (Photo by Author).
This year, believe the hype; this month’s Geminid meteor shower is a sure bet. This shower is one of the few dependable ‘old faithful’ meteor showers of the year. Peaking on the night of December 13th-14th, this year’s apparition sees a well placed northern radiant rising high in the northeast as the first quarter Moon sets about midnite local. The Geminid stream radiates from very near the bright star Castor in the zodiac constellation Gemini the Twins and typically produces up to 100 to 120 meteors per hour. If you are placed in mid-northern latitudes, you may see some activity shortly after sunset, but the real meteoritic action will begin after midnite. Think of a car driving at night in a snowstorm, not a stretch in the depths of the northern hemisphere winter. Looking forward into your high beams you get the cool vintage “Star Trek” effect, as you and your vehicle plow headlong into the stream of snowflakes. Think of the flakes as meteors and the car as the Earth; we face headlong into the meteor stream after midnight, and hence see more flashing meteor trains. The Geminids present several swift movers and fireballs, and the darker skies you have access to, the more you’ll see. Be sure to dress warm (it is winter out there!) and make a point to count and record your observations. Meteor shower observing is one of the few remaining scientific endeavors that remains low tech. Also, don’t forget to participate in the #meteorwatch via Twitter! This shower has a broad peak, and will be active the week of December 12th until the 18th, when the solstice-centered Ursids become active. In fact, there are some indications that the Geminids have been increasing in activity over the past decade, and certainly there’s a lot of material out there. The predicted peak centers on 5:00 AM UTC, just past midnite Tuesday morning from the US East Coast. And if that weren’t enough, it’s one of the last meteor showers with the Moon placed below the horizon until 2012; only the Quadrantids and Giacobinids have the same favorable geometry in 2011. Good luck, and be sure not to miss this unique meteor shower!
The astro-term for this week is the Yarkovsky Effect. The parent body that produces the Geminids, 3200 Phaethon, harbors somewhat of a mystery. Discovered in 1983, this space rock has been identified as the source of the Geminid meteor stream.NASA researchers estimate a massive amount of material exists, more than 100 times that of the average meteor stream. Unlike most streams that emanate from comets, however, 3200 Phaethon is an asteroid. Or is it an inactive comet? The mystery deepens, as the color of this strange rock is very similar to another asteroid, Pallas. 3200 Phaeton’s path sees it passing within Mercury’s orbit every 1.4 years, which brings it well within the realm of the Yarkovsky effect. This is the tiny bit of momentum imparted on a rotating body as it re-radiates photons absorbed from the Sun. On large bodies the effect may be negligible, but on tiny asteroids it can produce major changes in orbit over time. In fact, employing the Yarkovsky Effect by changing the reflectivity of an Earth-crossing asteroid is one way of possibly deflecting a lethal space rock. Is 3200 Phaethon a dormant comet or the remnant of an asteroid belt break up? This is one worldlet that definitely begs future exploration.
by VirtualAstro | Dec 8, 2010
Reports are coming in that people from all over the UK saw a very bright object streak across the sky!
The Object was reported to be incredibly bright with a flash which lit up the ground and then the object streaked across the sky, leaving a bright green tail in its wake!
Unfortunately I missed this amazing spectacle(drat!!!) and the first I herd of it was from a call I received from the BBC. BBC Radio 5 Live have had hundreds of text messages and calls from people who saw the event, and called me to ask if i new what it was?
The object was most definately a Meteor and would be refered to as a fireball or bolide (an incredibly bright fireball). What also makes me say this was a bolide is the apparent brightness duration (over 5 seconds) and its green tail. Typical of past bolide sightings from around the world.
Many people who saw the object reported a green tail and the reason for this is, the material the meteor is composed of oxidizes as it burns up. Most meteors are metalic and composed of iron with other trace metals. In tonights meteor's case it more than likely contained copper which is green when it oxidizes.
Reports of direction are sketchy, some people say south to North and Some say East to West, so we are unsure of its point of origin at present. Could it be an early chunk of the Geminids Meteor shower which peaks in the morning of the 14th December and is best seen the night before on the 13th through to the small hours? or is it a sporadic meteor? (please see below for mor explanations on what are meteors)
Please report your sightings on twitter using the hastag #meteorwatch or #meteor and join in with the Geminid Meteorwatch on Twitter on the evening of the 13th December 2011 Will it be a shower to remember?
I may be on BBC Radio 5 live at 11 – 11:30pm and will be discussing tonights fireball live from the Astrobunker.
What are Meteors?
Meteors are usually dust or sand grain sized pieces of rock which speed through space up to tens or hundreds of kilometers a second and when they enter the Earths atmosphere they burn up, creating bright or brilliant streaks across the sky.
Often referred to as "Shooting Stars" they can be seen randomly on most clear evenings and can be few or far between, these are called "Sporadic Meteors". A sporadic meteor can appear anywhere in the sky and from any direction.
You may be lucky enough and by chance to see larger sized pieces of debris burn up in the atmosphere causing very bright and enduring meteors, often referred to as "Fire Balls" or "Bolides". These are quite a sight and can last for several seconds in some cases.
Several times a year and on specific dates we have "Meteor Showers". A meteor shower is usually the left over debris from the tail of a comet which has in the past, passed through the Earths orbit or orbital plane around the sun. Because we know where and when these encounters happened we can accurately predict when the Earth will pass through the debris trail.
Some meteor showers only produce a hand full of meteors per hour and some produce up to a hundred or more meteors per hour. This is known as a "Zenithal Hourly Rate" or ZHR. We can also predict where in the sky or from what direction the meteor shower will come from, this is called the "Radiant".
A meteor shower will get its name from the constellation of stars the radiant occurs in, e.g. Perseids (Perseus), Leonids (Leo) and Geminids (Gemini) etc. A meteor is not to be confused with a "Meteorite" which is a meteor which has struck the surface of the planet, often very small pebble or stone sized.
Meteorites in extreme cases can be large, anything from the size of a football to many hundreds of Meters or Kilometers across. The Meteorite suspected of wiping out the Dinosaurs 65 million years ago, is estimated to have been 6 kilometers in diameter!
by Steve Owens | Nov 23, 2010
The final meteor shower of 2010 is the Geminids, the peak of which falls on the night of the 13/14 December 2010. The Geminids is described by the IMO as “one of the finest, and probably the most reliable, of the major annual showers presently observable”, and this year’s shower is set to put on a good show. (You can read the IMO’s rather technical summary of the 2010 Geminids here: http://www.imo.net/calendar/2010#gem)
It won't look like this
The predicted Zenith Hourly Rate (see my previous post about ZHR and what it actually means here) is around 120. Although the peak is predicted to occur around 1100 on 14 December, it should happen some time between 1840 on 13 December and 1600 on 14 December 2010. The best time for the peak to occur for stargazers in the UK would be between 0030 and 0600 on 14 December, after the Moon sets but before twilight begins.
The radiant for this shower is actually quite favourable, and if you wait till the Moon sets at around 0030 on 14 December then the only light pollution limiting your view will be man-made. If you observe before the Moon sets then you will lose a few of the fainter Geminids in its glow, but it’s only a first quarter moon, and so will only really have an impact if you’re observing from very dark skies.
Let’s use the equation relating ZHR to actual observations of meteors to work out how many you might see:
Actual Hourly Rate = (ZHR x sin(h))/((1/(1-k)) x 2^(6.5-m)) where
h = the height of the radiant above the horizon
k = fraction of the sky covered in cloud
m = limiting magnitude
In the case of the 2010 Geminids, if observed from the UK, h = 45 degrees. Let’s assume you have clear skies (haha) with k = 0.
The number of Geminids you can expect to see from a variety of observing sites is as follows:
For very light polluted sites, such as city centres m = 3, and therefore you can expect to see only around 8 meteors per hour.
In suburban skies near a city or town centre m = 4, and you’ll see around 15 meteors per hour.
In rural skies where m = 5, you’ll see 30 meteors per hour.
Under very dark skies, where m = 6.5 (i.e. where there is no or negligible effect of light pollution, like in Galloway Forest Dark Sky Park) you’ll see up to 85 meteors per hour, once the Moon sets. A first quarter moon will impose a limiting magnitude, even at a very dark site, of around 6, in which case you’ll see a slightly reduced 60 meteors per hour.
Remember, all of these numbers assume perfectly clear skies. If half your sky is cloudy, cut these numbers in half!
How many Geminid meteors will I see?
| Where are you observing from? |
Limiting magnitude |
Number of Geminids per hour |
| A very light polluted city centre |
3 |
7 or 8 |
| Suburban Site |
4 |
15 |
| Rural Site |
5 |
30 |
| Dark Sky Site |
6.5 |
85 (after the Moon sets at 0030) |
If you fancy a good view of this spectacular meteor shower, then head to Galloway Forest Dark Sky Park, where we have an evening of talks and meteorwatching planned, weather permitting!
Originally posted by Steve Owens (@darkskyman) on his blog Dark Sky Diary Pursuing darkness in an increasingly bright world
