My Dark Sky

A meteoroid is a solid object – usually are dust particles or debris from comets – that floats around in space.

When meteoroid enters and burns in the Earth’s atmosphere, the visible streak in the sky is known as a meteor.

A meteorite is the meteoroid that survives and reaches Earth’s surface.

An increase in the number of meteors at a particular time is called a meteor shower.

If we trace back the path of the meteors during a meteor shower, we will find that all the meteors seem to originate from a point in the sky. This point is known as the radiant.

Zenith Hourly Rate (ZHR) of a meteor shower is the number of meteor an observer would see in an hour under a dark sky with limiting magnitude of 6.5 and if the radiant was in the zenith. In reality the rate which can effectively be seen is always lower as the radiant is closer to the horizon and it also depends on the local weather condition.

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Meteors appear as fast-moving streaks of light in the night sky that usually will only last about a second or two. They are commonly referred to as “falling stars” or “shooting stars”.  Sometimes meteor may even leave a trail behind.

The vast outer space is actually not empty as it seems to be; there are a lot of dust particles, tiny grains of sand and ice floating around in space. When these particles or meteoroids come too close to Earth, Earth’s gravity will pull them into the atmosphere, at a speed of 10 to 70 kilometres per second, to produce streaks of light in the night sky known as meteors.

Most meteors will completely burn up in the atmosphere at an altitude of 100 kilometres.  However, some bigger chunk of meteoroids may survive the trip through the atmosphere and reach the ground. These remnants are known as meteorites.

What is Meteor Shower?

On a normal night we can typically see a few meteors per hour. This type of meteors is called the sporadic meteors. However, at certain times of the year the rate of observable meteors is much higher. These periods are called meteor showers.

Most meteor showers have their origins with comets. Comets are primarily composed of ice and dust and when they approach the Earth, the Sun’s heat will evaporate the ice and they will shed an icy, dusty debris stream which is then distributed along their orbits. When our Earth passes through a comet’s orbit, these left-over comet debris will “bombards” Earth and causes the rate of meteors increases.

Due to Earth orbit around the Sun, we will roughly be at the same location in space every year. Hence meteor showers will occur almost the same time each year when Earth crosses the comet’s orbit. The different between the yearly showers is related to how close the comet’s orbit to ours and how long ago the debris was ejected.

Why is Meteor Shower named after Constellation?

During a meteor shower, if we observe carefully, we will find that the path of the meteors seem to radiate from a point in the sky. This point is called the radiant of the meteor shower and is just a perspective effect. Actually all the meteors enter the Earth’s atmosphere in a parallel path, but from our Earth perspective, the meteors appear to come from the radiant.  This is just the same as parallel railway tracks seem to converge to a point far far away.

Meteor shower is named based on the location of their radiant. For example, if the radiant is located in constellation Perseus, the particular meteor shower is known as the Perseids. If there is more than one meteor shower in a constellation, then the shower is named after the bright star nearest to the radiant. For example, the Eta Aquarids and Delta Aquarids meteor shower are both from the constellation Aquarius.

How to observe Meteor Shower?

All you need to enjoy a meteor shower is just your naked eyes. No equipment is needed!

To observed, find a group of at least 5 people, go to a SAFE, dark and unobstructed site, away from city light or any man-made light polluter. The darker the site, the more dimmer meteors is going to be visible (although in the end the number of meteor visible is strongly dependent on the weather).

Although the meteors are said to come from the radiant, meteors actually can be seen all over the night sky, not just only in the direction of the constellation. It is just that when you trace back the route of the meteors, they seem to converage to a point, i.e. the radiant. So, don’t stare right at the radiant; you surely will miss out a lot of meteors over your head.

To make yourself comfortable, bring along mat to lie down. Lying down flat on the ground is the best position so that we can cover the maximum area of the sky. This position is a bit “dangerous” though, because this is also the best position to fall asleep and the next thing you know might be the Sun rising. Try chit-chatting with friends, this will help to keep you awake, but make sure that your eyes are glue to the sky; you won’t want to miss the show.

Sometime it may be cold in the middle of the night, so a jacket may come in handy. Better still if you can prepare some food and drinks to fill up your empty stomach during the night. Mosquito repellent may also be something useful to bring along.

Summary:

• Find a dark, safe and unobstructed observing site.
• Bring along star chart, red-light torchlight, food & drinks, jacket, ground-mat or sleeping bag.
• When you reach the observing site, find a place that is not easily disturbed by others.
• Check your direction and use the star chart to find the location of the radiant in the sky.
• Find a comfortable position to sit or lie down while waiting for the meteor to appear. The best position is to lie down flat on the ground so that you will cover the maximum area of the sky.

Why there are more Meteors after Midnight?

When Earth orbit around the Sun, basically meteoroids enter the Earth’s atmosphere from all direction. The velocity of Earth around the Sun is about 30 km/s and the meteoroids’ velocity is averagely 40 km/s.

Refer to the diagram, before midnight, the meteors are catching up with Earth from the back, hence the velocity of the meteor will be the different between the Earth’s orbital velocity and the meteoroids’ velocity, which is roughly 10 km/s.

After midnight, the meteors are heading straight to Earth, so the velocity now is the total of both the Earth and meteoroids’ velocity, which is equal to 70 km/s.

Hence, before midnight, only those meteoroids moving faster than Earth will catch up, so the amount of meteor seen is lesser. Also, the velocity of the meteor is lower before midnight, so the meteor will be dimmer during this time.

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Click here for handout on Meteor Shower in both English and Chinese. You are welcome to use it as teaching or handout material to the public.

~ by thChieh on May 2, 2008.

Posted in General Astro Talk
Tags: Meteor shower