Welcome to the Slashdot Beta site -- learn more here. Use the link in the footer or click here to return to the Classic version of Slashdot.

Thank you!

Before you choose to head back to the Classic look of the site, we'd appreciate it if you share your thoughts on the Beta; your feedback is what drives our ongoing development.

Beta is different and we value you taking the time to try it out. Please take a look at the changes we've made in Beta and  learn more about it. Thanks for reading, and for making the site better!



Ask Slashdot: How To Pick Up Astronomy and Physics As an Adult?

idji Local Library (233 comments)

Go to your local library and look at that 520's for Astronomy and the 530's for Physics. I guarantee you will find a beautiful modern book at esactly your level there.

4 days ago

Brown Dwarf With Water Clouds Tentatively Detected Just 7 Light-Years From Earth

idji Re:Occam's razor. (85 comments)

No, Astronomers have asked the WIMP vs MACHO question for many decades now, and WIMPs are winning.

Occam's Razor has always been applied here, and that is why it is still an open question, because the simple and obvious answer (MACHO) is not working and extraordinary evidence is being found, eg the Physic's Nobel Prize 2011.

This article is not about MACHO vs WIMP. It says they found a nearby MACHO with water vapor, and that is very interesting for life questions, not dark matter questions.

about a month ago

Google Wants To Test Driverless Cars In a Simulation

idji Re:Simulations are limited by imagination (173 comments)

You are right that real life is far more creative, but that is not the point being argued here. The point here is that simulation is a better testing environment than a test track. The test track will have much less creative scenarios than simulation because they are so much harder to stage. A test track will not test scenarios that people didn't think of. The simulator is a much better environment to test dog+stop sign+rain - try doing that on a track. Put some creative people in the simulator and they'll also test elephant+storm+headlight failure.

about a month ago

The Quiet Before the Next IT Revolution

idji Wrong (145 comments)

No, you IT people are no longer the great revolutionists - your time is gone. You are now just plumbers, who need to fix the infrastructure when it are broken. Other than that, we don't want to hear from you, and we certainly don't want your veto on our business decisions - that is why a lot of us business people use the cloud, because the cloud doesn't say "can't work, takes X months, and I need X M$ to set it up", but is running tomorrow out of operational budget.

about a month ago

The Technologies Changing What It Means To Be a Programmer

idji Re:Another Silver Bullet? I don't think so... (294 comments)

The most useful programming skills I learnt were in 1982 on the Sinclair ZX Spectrum. Nothing I learnt then has become irrelevant, just the languages changed. I give programming courses and am amazed how 21st century programmers are missing the basics and cannot write algorithms.

about a month ago

Rosetta Achieves Orbit Around Comet

idji Re:In Orbit? (54 comments)

The comet 67P has a mass of 3.14E12 kg
Today the comet is 186,444,271 km from the Sun Where is Rosetta?
Using F=GMm/R^2, the Sun's gravity on Rosetta is equal to 67P's gravity on Rosetta at 700m from the center of Rosetta on 6 August 2014, which means that Rosetta will never really be completely within 67P's field. (At Perihelion on 13 Aug 2015, 67P's gravity field will be as strong as the Sun's only 250m from the centre) However, now that Rosetta is in the same orbit as 67P we can mostly disregard the Sun's gravity and the elliptical path that Rosetta and 67P now share as of today. (Earth's pull on Rosetta is at least a million times weaker than the Sun's pull - so forget any influence from the Earth's mass.)

The "orbits" at 100km are called hyperbolic because Rosetta is not trapped in 67P's gravity well since the gravity is so weak and because Rosetta is still moving FAST at 1 m/s. But this hyperbola is so weak it is effectively a straight line.
Rosetta will turn 60 degrees after every 100 km of a hyperbolic path to make a triangular "orbit". This triangular path cannot be called an orbit because it is not a conic section, nor is the comet at a focal point of the conic section Kepler's First Law.

These "straight"/"hyperbolic" paths of 100km and 50km are deliberately done for two reasons:
-to calculate exactly the gravity field of the comet, because it is clearly not a uniform sphere. They will likely use radar&cameras to continuously measure the precise distance to the comet
-to keep in front of the comet to avoid its coma and tail.
After these maneuvers, Rosetta will go into a 30 km "orbit", so that the task of mapping 80% of the surface all happens from the same distance. This orbit is not natural and will be powered because a natural 30km orbit of 67P takes 26 days.

Here's how to calculate the natural circular orbits for 67P (it won't be circular, because of the crazy shape, but close enough). Kepler's 3 Law gives us
T^2=4pi^2/GM*r^3. 4pi^2/GM=0.19 for this comet. G=6.67×1011 N(m/kg)2
if r=30km=3e4m, the natural orbit would have a period of T=2.3e6 seconds=26.11 days
If r=2.5km, the natural orbit would have a period of T=15 hours
If r= 5km, the natural orbit would have a period of T=1.77 days
If r= 100km, the natural orbit would have a period of 159 days So I could imagine that when Rosetta gets within 5km it is mostly using the natural orbit and hence saving fuel.

about a month and a half ago

Gmail Recognizes Addresses Containing Non-Latin Characters

idji Re:Dammit this is a terrible idea (149 comments)

It would be easy to WARN a USER if the name contains mixed alphabets or diacritics that differed from the user's browser's preferred language. Each Unicode Character has a name eg "Greek Upsilon With Hook Symbol", or "Latin Capital Letter R", or "Cyrillic Capital Letter Es With Descender", "Arabic Letter Qaf", or "CJK Ideograph" for Chinese/Korean/Japanese.

about a month and a half ago

Tesla and Panasonic Have Reached an Agreement On the Gigafactory

idji Re:What makes this a gigafactory? (95 comments)

go and look at the PICTURE on TFA and you will see they want to make 35GWh/yr (35 GIGAWATT HOURS PER YEAR) of cells by 2020. So Gigafactory is quite appropriate.

about 2 months ago

NIF Compresses Diamonds With 50 Million Atmospheres of Pressure

idji Re:Car analogy? (81 comments)

during the impact.

about 2 months ago

Sand-Based Anode Triples Lithium-Ion Battery Performance

idji Re:Launch date (60 comments)

please unsubscribe from "news for Nerds". We are here for news, and we are nerds, we want new ideas.

about 2 months ago

Physicists Spot Potential Source of 'Oh-My-God' Particles

idji Re:Ooh, ooh, I have a bogus theory (144 comments)

because the annihilation particles are all well known and have MUCH less energy than these particles.

about 2 months ago

Police Using Dogs To Sniff Out Computer Memory

idji Re:Amazoing (415 comments)

lots of chemicals are used in making/etching circuit boards and then there is the tin&flux in solder. I am sure they have very clear smells.

about 2 months ago

Air Pollution Can Disrupt Pollinating Insects By Concealing the Scent of Flowers

idji Re:Easy fix (67 comments)
Bees need to fly twice around the word (50,000 miles) and visit 2.6 million flowers to make 1 pound of honey.
Your robots are not coming to save us any time soon.

about 3 months ago

New Chemical Process Could Make Ammonia a Practical Car Fuel

idji Re:Now I'm confused ... (380 comments)

They have just found a cheap way to crack NH2 to N2 and H2 and are excited about that in combo with simpler fuel storage and transport - they are not focusing on the energetics of H2 or NH3 generation with the Haber-Bosch process here.
The point here is that to store Hydrogen you need 10,000 psi ( and Ammonia only needs 250 psi in a plastic container (
They are looking at the following problem
and have worked out that they can do
H2O+Energy->H2,+N2+Energy->NH3->NH3-Storage->H2 +N2 without NOx->FuelCell->Electricity +H20
and what they are excited about is that NH3 storage and transport is a known and solved problem industrially and NH3 cracking is now cheap and clean. Now someone just needs how to work out H2O->H2->NH3 using solar and the problem is solved.

There is also the other issue that a H2 leak is benign or a quick fireball and that an NH3 leak will eat the noses and lungs of everyone nearby....

about 3 months ago

Elon Musk: I'll Put a Human On Mars By 2026

idji Re:At least Elon has the right goal (275 comments)

Going to Mars won't save humanity, but working out how to live away from Earth could, even if that means learning on Mars how to live under the Earth to survive the 1450's imps, 1950's Apocalypse Godzillas and 2010's Biotech Zombies...

about 3 months ago

Unicode 7.0 Released, Supporting 23 New Scripts

idji Re:Why emoji? (108 comments)

If the emoji are standardized in Unicode, then it will be easier for any kind of software to support them.

about 3 months ago

Dump World's Nuclear Waste In Australia, Says Ex-PM Hawke

idji Re:Commodity of the future (213 comments)

Look at Safety Advantage #11 Destruction of existing long lived wastes in LFTR What is wrong with my periodic table?

about 4 months ago



How did the Moon move so far in one day?

idji idji writes  |  more than 5 years ago

idji (984038) writes "Saturday morning I woke up at 7am and looked out my south-facing window (48N) and saw the moon BEHIND a tree. Sunday morning I woke up at 8am and looked out my south-facing window. No surprise that the Moon was there. But what blew me away was that the moon wasn't behind the tree, it was ABOVE the tree, about 8(!) moon-diameters higher in the sky. I have studied planetary science, the orbit of Mercury and can derive Kepler's 3 Laws from F=ma on the back of an envelope but I didn't have an explanation for what I saw — I skipped that observational Astronomy class at university — too much trigonometry. Orbit of the Moon didn't help me either. Can any one out there explain how the Moon was 8 diameters higher in the school AFTER 25 hours?"

idji idji writes  |  about 8 years ago

idji writes "Astronomers meeting in the Czech capital have voted to strip Pluto of its status as a planet. About 2,500 were in Prague for the International Astronomical Union's (IAU) general assembly. Pluto has now been demoted to a pluton — transneptunian — kuiper belt object — whatever you want to call it."


idji has no journal entries.

Slashdot Login

Need an Account?

Forgot your password?

Submission Text Formatting Tips

We support a small subset of HTML, namely these tags:

  • b
  • i
  • p
  • br
  • a
  • ol
  • ul
  • li
  • dl
  • dt
  • dd
  • em
  • strong
  • tt
  • blockquote
  • div
  • quote
  • ecode

"ecode" can be used for code snippets, for example:

<ecode>    while(1) { do_something(); } </ecode>