Science: The Stats - The illusion of the ecliptic

As comet Hale-Bopp fades from view, Heather Couper and Nigel Henbest explain why our solar system is in the shape of a disc with the Sun at its centre

For the past month and more, we've perhaps become rather blase about having a beautiful comet gracing the evening sky. Comet Hale-Bopp has lived up to all our expectations, retrieving the reputation of comets and astronomers alike after the fizzlers of Kohoutek and Halley (though, to be fair, no astronomer expected a great show from Halley; it was a victim of media hype). We've been blessed with the comet of a lifetime.

Alas, it's now on its way out. Hale-Bopp has been fading over the past few weeks, and is now heading down south and into the evening twilight glow. It's good news for people in the southern hemisphere, who have missed out on the comet at its best, and will now get to see at least something of Hale-Bopp before it heads back into obscurity in the far reaches of the outer solar system.

As you can see from the chart, early in May Hale-Bopp's track takes it across the imaginary line we've marked as "path of the moon and planets" - technically known as the ecliptic. Comets are one of the few denizens of the solar system that seem to stray from this track through the sky.

If we could strip away the bright daytime sky, we'd see that the sun, too, is always found on the ecliptic. During the course of the year, it seems gradually to trundle around the sky. That's why the names of the constellations along the ecliptic, such as Taurus, Gemini, Cancer, Leo and Virgo, sound familiar. About 5,000 years ago the Sumerians invented these constellations - or "signs" - as markers around the ecliptic, one for each month, just as we have numerals around a clock dial. Because so many of the constellations represent animals, the Greeks called them the zodiac, meaning "band of animals".

The Sumerians also believed that the influence of the sun, moon and planets changed as they moved from one zodiacal sign to the next. The whole basis of astrology is, of course, nonsense - as you could readily see if the stars really were visible during the day. Because the earth's axis swings around gradually in space, the sun's position has changed since astrology was invented. If you are born in early May, astrologers will tell you that your "sign" is Taurus. You'd in fact see the sun in Aries on your birthday.

Why this magnetic attraction for the ecliptic? It's all a matter of geometry. The earth and the other planets circle the sun in a flat disc; seen from outside, the orbits of the planets lie in the same plane. This is a relic of the planets' birth. They condensed from a disc of gas and dust shaped like a CD, with the sun in the hole in the middle. Living within this flat distribution of planets, we see their orbits edge on, so they seem to follow the same line through the sky.

Only in its outer parts did the early constituents of the solar system stray from the thin and narrow. Pluto has an orbit tilted by 17 degrees from the other planets. So this month, for example, Pluto lies in the constellation Ophiuchus, well away from the ecliptic. Comets such as Hale- Bopp start their life further out still, in a vast, spherical cloud around the sun called the Oort Cloud (after the Dutch astronomer Jan Oort who predicted its existence). Hale-Bopp's orbit is tipped at right angles to that of the planets; its swoop over the earth's north pole, before diving down to the ecliptic, was responsible for the grandstand view we've been having.

Diary (all times BST, 24-hour clock)

6 21.47 new moon

14 11.55 moon at first quarter

22 10.14 full moon; Mercury at greatest western elongation

25 Pluto at opposition

29 08.52 moon at last quarter

Suggested Topics
PROMOTED VIDEO
Have you tried new the Independent Digital Edition apps?
News
ebooksAn unforgettable anthology of contemporary reportage
Life and Style
food + drink
Life and Style
Shoppers in Covent Garden, London, celebrate after they were the first to buy the iPhone 6, released yesterday
tech
News
Liam Payne has attacked the media for reporting his tweet of support to Willie Robertson and the subsequent backlash from fans
peopleBut One Direction star insists he is not homophobic
Life and Style
healthFor Pure-O OCD sufferers this is a reality they live in
Arts and Entertainment
A bit rich: Maggie Smith in Downton Abbey
tvSeries 5 opening episode attracts lowest ratings since drama began
Arts and Entertainment
Ben Affleck stars as prime suspect Nick Dunne in the film adaptation of Gone Girl
filmBen Affleck and Rosamund Pike excel in David Fincher's film, says Geoffrey Macnab
Life and Style
fashion
News
news
News
people
Travel
Warner Bros released a mock-up of what the new Central Perk will look like
travel
Arts and Entertainment
Lena Dunham
booksLena Dunham's memoirs - written at the age of 28 - are honest to the point of making you squirm
News
i100
Arts and Entertainment
Jake Quickenden sings his heart out in his second audition
tvX Factor: How did the Jakes - and Charlie Martinez - fare?
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs Media

Senior Account Executive / Account Executive

£25 - 30k (DOE) + Bonus & Benefits: Guru Careers: We are looking for an Accoun...

Account Manager / Sales Account Manager / Recruitment Account Manager

£25k Basic (DOE) – (£30k year 1 OTE) : Guru Careers: We are seeking a bright A...

Resourcer / Junior Recruiter

£15-20k (DOE) + Benefits / Bonus: Guru Careers: We are seeking a bright R...

Web Designer / Digital Designer

£25 - 40k (DOE) + Excellent Benefits: Guru Careers: We are seeking a Web Desig...

Day In a Page

A roller-coaster tale from the 'voice of a generation'

Not That Kind of Girl:

A roller-coaster tale from 'voice of a generation' Lena Dunham
London is not bedlam or a cradle of vice. In fact it, as much as anywhere, deserves independence

London is not bedlam or a cradle of vice

In fact it, as much as anywhere, deserves independence
Vivienne Westwood 'didn’t want' relationship with Malcolm McLaren

Vivienne Westwood 'didn’t want' relationship with McLaren

Designer 'felt pressured' into going out with Sex Pistols manager
Jourdan Dunn: Model mother

Model mother

Jordan Dunn became one of the best-paid models in the world
Apple still coolest brand – despite U2 PR disaster

Apple still the coolest brand

Despite PR disaster of free U2 album
Scottish referendum: The Yes vote was the love that dared speak its name, but it was not to be

Despite the result, this is the end of the status quo

Boyd Tonkin on the fall-out from the Scottish referendum
Manolo Blahnik: The high priest of heels talks flats, Englishness, and why he loves Mary Beard

Manolo Blahnik: Flats, Englishness, and Mary Beard

The shoe designer who has been dubbed 'the patron saint of the stiletto'
The Beatles biographer reveals exclusive original manuscripts of some of the best pop songs ever written

Scrambled eggs and LSD

Behind The Beatles' lyrics - thanks to Hunter Davis's original manuscript copies
'Normcore' fashion: Blending in is the new standing out in latest catwalk non-trend

'Normcore': Blending in is the new standing out

Just when fashion was in grave danger of running out of trends, it only went and invented the non-trend. Rebecca Gonsalves investigates
Dance’s new leading ladies fight back: How female vocalists are now writing their own hits

New leading ladies of dance fight back

How female vocalists are now writing their own hits
Mystery of the Ground Zero wedding photo

A shot in the dark

Mystery of the wedding photo from Ground Zero
His life, the universe and everything

His life, the universe and everything

New biography sheds light on comic genius of Douglas Adams
Save us from small screen superheroes

Save us from small screen superheroes

Shows like Agents of S.H.I.E.L.D are little more than marketing tools
Reach for the skies

Reach for the skies

From pools to football pitches, rooftop living is looking up
These are the 12 best hotel spas in the UK

12 best hotel spas in the UK

Some hotels go all out on facilities; others stand out for the sheer quality of treatments