Monthly Archives: October 2012

Tides – the origins

It started off with mum and I watching the floods brought by Sandy on CNN and mum asked what’s the cause for waves. Well wind caused Sandy’s waves. But, Daily tides, high and low, are caused by the moon.

The accepted theory is the moon exerts a force of gravity on earth as the moon rotates earth (both the moon and earth rotate counter clockwise, but earth’s rotation is much faster). The parts of earth passed by the moon are pulled towards the moon, and when water is pulled, tides are formed! Yayy! Sounds easy right.

But, there comes this picture

– with the theory that the moon pulls the side closest to it the most, pulls the middle of the Earth with a medium force and the side furthest away with the least force. So the bulge of water on the left of the pic is pulled the least, appearing as a bulge as Earth is moved towards the right.

The sun comes into the picture as well, also exerting a pull on water. But, the moon’s pull beats the sun. Though the suns is massive, it’s too far away (F=GMm/r2, Newton’s Law of Gravitation) And if the moon and sun are on opposite directions to Earth, high tides become lower.

But then again, if the moon pulls pulls parts of Earth with different distances from it with different forces, each part would have a different acceleration. F=ma. The parts closest to the moon would accelerate the most (in the pic above, it would be the right end of Earth) and the opposite end (the left), the least. And so, with one end gaining acceleration compared to the other, Earth would be stretched and eventually pulled apart.

So, there has to be a restraining force. Earth’s gravity and its tension? Or a centrifugal force? Now what about a centripetal force?

If you want to end up confused (like myself), I suggest you read:
1. Tides and Centrifugal Forces by Paolo Sirtoli. Lots of physics explained here – with equations.
2. Tidal Forces and the Effects on the Solar System by Richard McDonald. Simpler physics and much easier to understand.
3. Tidal misconceptions by Donald. E Simanek. This is the most comprehensive to me, though let me warn you its quite long.

And then there’s the belief that tides don’t form on the equator. But I’ve seen it for myself, they do form! In fact, they’re semidiurnal tides (two equal high tides and low tides in a day). One high tide on each of the pair of “bulges” created. Check out ocean motion. to find out about types of tides at different latitudes on Earth.

I’ll keep you updated on tides once I piece everything together (and find some missing links, like I’m sure the moon’s orbit around Earth at at about a 5 degree angle to Earth’s ecliptic would have an effect as well). Let me know if you’ve got any ideas of your own!

And mum has two questions of her own:
1. Why isn’t paper pulled towards the moon when its so light.
To me, it is, but the effect is so small that you’ll never realize it.
2. Why do you get the highest tides on the full moon. This, I don’t know. So let me know if you do pleaseee.

ps. Here’s some videos on movements of the moon and of Earth.

Some cool stuff here!

Categories: Misc, Physics, Solar System, Uncategorized | Tags: , , , , , , , , | 1 Comment

Star Party Time!

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Was looking through some old pics and I stumbled upon this bunch.

Our stargazing night to catch Saturn! Sometime around mid this year I believe. And we had late night park-walkers joining in! The clouds were coming in and we were keeping our fingers crossed. And we were lucky to catch Saturn in time! It was the first sight of the beautiful planet – its rings and the Cassini Division – for many of us, including myself! And when the clouds rolled in, we packed up and headed off for some seriously unhealthy but the sweetest supper: “Roti Tissue”, paper thin pancakes cooked with a layer of condensed milk. Sounds good right.

One mystery though…Some of them said Saturn looked white, but to me, it was orange….

How’s Saturn like to you?

Categories: Amateur Astronomy, Misc, Observation Sites, Solar System, Uncategorized | Tags: , , , , , , , , , | 1 Comment

Clear Skies!

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It’s 4 AM and Jupiter, Mars, and Orion are right above my balcony. Whatta treat!!

Been such a long time since we’ve had clear skies and when it was clear last night, I just had to take the Dob out! – despite loosing much sleep…with upcoming exams, oh noo!

Btw, Jupiter doesn’t look nearly like the picture I took…after 1/2 and hour struggling to get (and focus) it into the field of view of the camera. You’ll actually be able to see its orange rings with a 24mm Plossl, and swirls on it with a 10 mm Plossl eyepiece! Mars was orange as usual. And the Nebula with its Trapezium, frozen in time. Bout time I get some filters – for some colour with the nebula and for better definition of the Great Red Spot. And I reallyyyy wanna see the Horsehead nebula on Orion!

Any thoughts on good filters? I’m thinking Baader’s Moon and SkyGlow filter – seems like a good all around filter for both sodium vapour wavelengths (light pollution) and for contrast.

Clear skies!

Categories: Amateur Astronomy, Misc, Observation Gear, Solar System | Tags: , , , , , , , , | 2 Comments

StarHopping – Old Skool Style

So, if you don’t have a PushTo, GoTo or a Telrad (like myself), this is whatcha gonna need for some old-skool style star-hopping.


Star Chart

Here’s a link to a free downloadable version which I’ve used for my observation nights with my club: SFA Star Chart. Star Charts map constellations, nebulae and galaxies, and they sometimes even indicate brightness! So, before you start observing, you’ll need a star chart to know what’s up in the sky tonight, decide which object to hunt for and use the pattern of stars to guide you there. Without star-hopping, locating an object is like looking for a needle in a haystack – especially if your eyepiece has a narrow field of view and a short focal length – talk about tunnel vision!


A Compass


The trick is to imagine your star chart as a sphere encapsulating Earth.

From the Equatorial Region

Check your Star Chart and lookout for today’s date (its on the horizontal axis). If you imagine a vertical line through the point which marks the date, that would be the view of the skies at the Celestial Meridien at 8 pm. Then on, just minus or add the time of your viewing to the Meridien (at 8 pm) and shift your focus (the imaginary vertical line) on the map – basically, aligning the Celestial Meridien to the current time. So, if you’re observing at 11pm, you would need to add 3 hours (11-8=3) to the 8pm Meridien, which means, looking 3 hours to the left of the imaginary 8pm Meridien line on the map. if you’re observing at say 7 pm, look an hour (8-7=1) to the right of the 8pm Meridien. Once you’ve approximated your vertical axis, look up to the skies and align it with what you see.

Now if you wanna see the Eastern sky, align yourself to face East – here’s where the compass comes in. Look at the left of the vertical line which matches your time of viewing and date. And if you want to see the Western sky, face West and look at the right of the imaginary line. You see, as Earth rotates, you’ll see new stars emerging on your East and other stars falling under the Western horizon.

It all may sound quite confusing. So, back to basics, imagine your chart as a sphere wrapped around Earth, and remember how Earth’s rotation makes the sky look like its moving. Check out: Why Stars Move

Next up, is using your hands. Once, you’ve located what you see on the chart, to hop to the right star, you’ll need to measure. On the chart, are “latitude lines” 10 degrees apart. How on earth would you know what’s 10 degrees apart in the sky. Well, make a fist and point it up to the sky, the length of a fist measures 10 degrees in the sky. The thumb, about 5 degrees. And if you open you hand as wide as possible, from the tip of the little finger to the thumb’s tip, 25 degrees.

Now, you’re all set!

Zoomed in Star Chart

ps. What’s the Celestial Meridien?It’s a¬†circle the passes through the north and south celestial poles, the zenith and nadir. Read: Measuring the Sky to find out more.

Categories: Amateur Astronomy, Observation Gear, Observation Tips, Stars | Tags: , , , , , , , , | 6 Comments

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