Sky Matters: Sirius will be visible in the middle of March

The Spring (or Vernal) Equinox takes place on March 20th, writes Niall Smith. 

It marks one of two days in the year when the nights and days are of roughly equal length across the world and when the Sun is directly above the Earth’s equator. In Ireland, “Spring” is generally regarded as starting on February 1st, in the UK it’s March 1st and in the US it coincides with the Spring Equinox! This lack of agreement on when Spring officially begins can cause some confusion – there’s an almost seven week difference – but in reality the definition is rather arbitrary.

Notwithstanding the debate about the onset of Spring, what if you were on another planet – would you still experience seasons? The answer is yes, but it varies widely. For example, both Jupiter and Venus have a very small tilt of their axis, a mere 3 degrees compared to 23.5 degrees for the Earth, and their orbit around the Sun is almost circular, meaning they are roughly at a constant distance and receive a constant amount of sunlight.

This combination means they experience no discernible seasons. By contrast, Mars has a larger tilt than the Earth, about 26 degrees, so its seasons are slightly more pronounced anyhow, but the effect is compounded because its orbit also takes it farther from the sun during its northern hemisphere winter making that a particularly cold time.

But the planet that probably suffers the most extreme seasons is Uranus, where each season lasts 84 years!

This giant planet has an axis tilted a whopping 98 degrees, so if you were at the northern pole you’d have 84 years of permanent sunshine during summer and 84 years of almost permanent night during the winter.

You can see Venus as an evening object until the middle of the month and then it begins to get lost in the evening twilight.

The planet is so bright at the moment that it may seem to be emitting “rays of light”, an effect caused by your eyes but one that nevertheless can seem very real. Mars is visible in the west after sunset, red-hued but unremarkable in brightness throughout the month.

Jupiter is the brightest morning object and it will form a beautiful triangle with the Moon and the star Spica on 14th. No need for telescopes or binoculars – use your eyes to soak up this free celestial spectacle.

It’s not too late to look for the brightest star in the sky, Sirius (the Dog Star), during March. In the middle of the month it is visible due south around 8pm and has an intensely blue-white colour.

Sirius never gets very high in the sky from Ireland and it ordinarily twinkles a lot. The twinkling of stars in general happens because the Earth’s atmosphere, especially near the horizon, is turbulent and distorts their light – the stars themselves do not twinkle.

If you look closely at Sirius you’ll see this twinkling takes on a rainbow of colours that ebb and flow randomly.

That’s the atmosphere acting as a kind of little prism.

If you were on the Moon the stars would not twinkle because the Moon has no atmosphere.

  • For more observing information see March’s SkyMatters Newsletter at www.bco.ie/skymatters or on the online version of the Examiner.)

Did you know?

Enceladus, one of Saturn’s moons, reflects about 90% of the sunlight that falls on it, making it more reflective than snow!

Even more intriguing is the possibility that beneath this moon’s icy crust there lies an ocean of water that may be rich in organic materials, the building blocks of life.



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