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Meteor Showers

Most meteors are just sand grain-sized objects hitting the atmosphere. Sometimes a big one the size of a pebble causes a chorus of "ah"s at a star party - the brilliant flash is quite stunning. Normally, when the skies are dark, we can see perhaps five sporadic meteors per hour.

On certain dates every year, we can see many more than that. That is because the Earth crosses an orbit of debris from a comet. All these meteors seem to come from the same spot in the sky because that is the direction the debris comes from. Hence meteor showers are named after the constellation where they seem to come from.

The time when the most meteors can be seen is about two hours before sunrise. In the morning our planet is facing the direction we are moving around the Sun.

Below is a list of the best meteor showers. The Perseids in August is the shower most often watched.

Shower Name Constellation it appears to come from
Dates
Date of peak of shower Number per hour if very dark sky
Eta Aquarids Aquarius Apr 19 - May 28 May 6 60
Lyrids Lyra Apr 21 - 22 Apr 22 10-20
Arietids Aries May 22 - Jul 2 Jun 7 54
Perseids Perseus Jul 17- Aug 24 Aug 12 90
Leonids Leo Nov 14- Nov 21 Nov 17 variable up to thousands
Geminids Gemini Dec 7 - Dec 17 Dec 14 120

Eclipses

The Moon orbits the Earth, but at a tilt of 5 degrees compared to the Sun. When the Moon's phase is new, its shadow usually misses the Earth, when the phase is full, the Moon usually misses our shadow. When the three directly line up, we have an eclipse.

During a lunar eclipse, about half the planet can see the full Moon get 100 times darker over a few hours. The Moon has a reddish glow because the light striking the Moon passes through the edge of the Earth's atmosphere.

Solar eclipses are less frequently observed and more dramatic. For most people, the Moon just covers part of the Sun. It's often not even noticed unless you look through special eclipse glasses, or watch the round dapples of light through trees turn into crescents.

But those in the narrow path of the Moon's full shadow witness one of nature's grandest spectacles. During the total eclipse phase, the sky gets dark, it gets cooler; bright stars can be seen and nocturnal animals wake up. You can watch the total phase with binoculars or your unaided eyes - the haunting corona which is hotter than the Sun, solar flares, and shadow bands on the ground. At any other time, it is not safe to look at the Sun without a proper solar filter that is known to be safe.

Eclipses are predicted many years in advance; and several companies and RASC Centres sponsor tours.

Another form of eclipse is a transit, when a planet crosses the disc of the Sun. Mercury will transit on May 9, 2016, Venus will transit on June 6, 2012 with a 105-year wait for the next transit after that.

Transits of planets orbiting other stars have been observed. It is a promising technique to find Earth-sized extrasolar planets. Many stars need to be continually observed to find the several hour long slight darkening caused by a transit of an Earth-like planet. If that repeats at regular intervals, a planet has been located. The Kepler Mission will watch many stars near the constellation Cygnus for several years to look for planets.

Comets

Comets are 0.1- to 40-km diameter objects made mostly of dust, ice, and dry ice. They make strange journeys around the Sun, spending years moving slowly far away, then coming closer for a quick pass, then returning back to the frozen outer reaches of the Solar System. However, some short-period comets return every few years.

When a comet gets closer to the Sun, more dry ice vaporizes and the comet releases more dust. The solar wind pushes the dust away from the Sun to make a tail.
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Some comets return at predictable times. Halley's comet was / will be near the Sun in 1910, 1986, and 2061.

Other comets just appear out of the blue. Comet McNaught was discovered in August 2006, was visible in Canada during early January 2007 (even during daytime), then it passed into the southern hemisphere with a stunning tail as shown here. Comets are named after their discoverers.

Who knows what comets will be discovered in the next few years?

Aurorae

Aurorae (also known as the Northern Lights and Southern Polar Lights) happen because of events on the Sun, which affect the Earth a few days later. The Sun always gives off particles, but at times, a solar flare gives off many more particles than normal. When these hit the Earth's atmosphere, the glow of aurora can be seen widely.

To take pictures of aurorae, use a tripod and the widest-angle lens you have at the highest ISO setting and widest aperture. An exposure of ISO 800 at f/5.6 for 30 seconds will show aurorae and their bright colours. Details of changing features show up better on short exposures.

For digital cameras, the automatic focus will not work for astronomical objects. Change to manual focus and turn the focusing ring to focus (while wearing any distance glasses) on a sharp distant object, such as a light a few hundred meters away. If such an object is not available, turn the focusing ring to the "infinity" setting.

Supernovae

Stars much larger than the Sun will eventually blow up in so-called "supernova" explosions which, for a short time, give off as much energy as the rest of their parent galaxies altogether.

Galaxies our size typically have about one supernova per century. The last bright Milky Way supernova was almost 400 years ago, although the Large Magellanic Cloud (a satellite galaxy to the Milky Way) had a supernova 1987. It was first reported by Ian Shelton, a Canadian astronomer. People in the southern hemisphere could see a new bright star where only a faint star had been seen earlier. After a few months, supernovae fade.

The Star Finder shows three stars large enough to become supernovae - Betelgeuse, Spica and Antares. One of them might already have exploded. They are all hundreds of light years away. It takes that many years for information to reach us by light, radio waves and gamma rays.

Supernovae are important for the history of the galaxy and planets, because heavy atoms like iodine and gold can only be made in supernovae. Some of the mass in the Sun and our Solar System and our bodies originated in supernovae.

One type of supernovae typically has the same brightness. Careful observation of these particular events allows astronomers to calculate the distance to far-away galaxies. Astronomers were very surprised to find that distant galaxies appear to be moving away from us much faster than expected. The cause of the "dark energy" driving this acceleration is still a mystery.

 
     

Future eclipses are listed in the RASC Handbook (comes with membership) and at a NASA site.

The SkyNews Magazine Web site will list any comet easily visible from Canada.
For more information about Meteors and when and where to look for meteor showers, visit these links.
The Web site Space Weather gives up-to-date information about flares, the solar wind, and the size of the aurora oval around Earth's poles.

 

© 2008 - Royal Astronomical Society of Canada