10Mar2018

MARCH EQUINOX Beginning Of Autumn

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There are two equinoxes every year – in March and September – when the Sun shines directly on the equator and the length of night and day are nearly equal. It means different things to each hemisphere.

March Equinox in Sydney, New South Wales, Australia is on Wednesday, 21 March 2018 at 3:15 am AEDT

The Sun Crosses the Equator

The March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north. This happens on March 19, 20, or 21 every year.

Northern Spring – Southern Autumn

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Equinoxes and solstices are opposite on either side of the equator. The March equinox is the spring (vernal) equinox in the Northern Hemisphere, marking the start of astronomical spring.

In the Southern Hemisphere, it is the autumnal  equinox, which marks the start of Autumn.

Why Equinox?

On the equinox, night and day are nearly the same length – 12 hours – all over the world. This is the reason it’s called an “equinox”, derived from Latin, meaning “equal night.” However, in reality, equinoxes don’t have exactly 12 hours of daylight.

What Happens on the Equinox?

The Earth’s axis is always tilted at an angle of about 23.4° in relation to the ecliptic plane, the imaginary plane created by the Earth’s path around the Sun.

On any other day of the year, either the Southern Hemisphere or the Northern Hemisphere tilts a little towards the Sun. But on the two equinoxes, the tilt of the Earth’s axis is perpendicular to the Sun’s rays, like the illustration shows.

Used to Measure Tropical Year

The March equinox is often used by astronomers to measure a tropical year – the mean time it takes for the Earth to complete a single orbit around the Sun. Also known as a solar year, a tropical year is approximately 365 days, 5 hours, 48 minutes and 45 seconds long.

The time between one March equinox and the next can vary by only a few minutes or by as many as 30 minutes each year.

For example, the time between the March Equinox in 2015 and the March Equinox in 2016 was 365 days, 5 hours, 44 minutes, and 56 seconds, while the same duration between the March Equinoxes in 2016 and 2017 will be 365 days, 5 hours, 58 minutes, and 36 seconds.

Celebrating New Beginnings

The March equinox has long been celebrated as a time of rebirth in the Northern Hemisphere. Many cultures celebrate spring festivals and holidays around the March equinox, like Easter and Passover.

TONIGHT’S SKY AT A GLANCE

Skywatching

Venus and Mercury are in the evening sky, but they are quite low to the western horizon and aren’t easy to spot. In the early morning sky, Jupiter, Mars and Saturn can be found stretched out in a line, crossing the beautiful Milky Way.

PLANETS

Mercury and Venus begin the month together low to the western horizon after sunset. Mercury is below and to the right of Venus, but since it’s the faintest planet, it will be hard to spot against the evening twilight sky.

On the 19th, the thin crescent Moon sits just above Venus and Mercury. Soon after, Mercury disappears below the horizon but Venus will be visible for a little longer.

Earth experiences the Autumn Equinox on Wednesday 21st. At 3:15am, out in the space, the Sun crosses the celestial equator heading north. Day and night are of equal length a few days later, on Saturday 24th.

This delay is partly because our atmosphere bends light from the Sun, and so, we see the Sun before it physically rises and continue to see it for a short while after it has set. This phenomenon is called atmospheric refraction.

Mars can be found in the eastern sky before sunrise. It begins the month sitting between Jupiter and Saturn, and just below the lovely constellation of Scorpius.

As the month goes by, Mars drifts towards Saturn and when the month ends the two will be quite close. The Last Quarter Moon is just to the left of Mars on the morning of the 10th.

Jupiter rises late in the evening and is high in the north by sunrise. It sits just ahead of the claws of Scorpius. The Moon passes by Jupiter on the mornings of the 7th and 8th.

Saturn is found in the east at sunrise, with Mars and Jupiter stretching out above it. The Moon sits to the left of Saturn on the morning of the 11th. Throughout the month, Mars approaches Saturn so that it sits directly above the ringed planet on the 31st.

METEORS

There are two small meteor showers this month that occur near the South Celestial Pole. The gamma Normids is due to peak around the 14th. This shower is centred on the yellow giant star, gamma Normae in the constellation of Norma, the level.

The second shower is the delta Pavonids, which peak in early April, but will start to appear from the 21st. This shower occurs in Pavo, the peacock. The best time for viewing meteor showers is generally between midnight and dawn.

STARS & CONSTELLATIONS

The constellations of Orion and Taurus can be found in the northwest after sunset. Taurus contains the beautiful Pleiades or Seven Sisters, a small cluster including many young blue giant stars.

The brightest star in our night sky, Sirius (Canis Major) is nearly overhead at sunset. Its partner, Procyon in Canis Minor, is high in the north. The twin stars of GeminiCastor and Pollux, lie low in the north-west while Regulus, in Leo, is low to the north-east.

The constellation of Virgo rises in the east after sunset. Sitting above Virgo is the kite-shaped group of stars that form Corvus (the crow).

Crux (or the Southern Cross) is now beginning to climb up to its autumn position – lying on its side in the south-east.

GLOBE at Night Can you see the stars?

Join thousands of people world-wide hunting for stars. GLOBE at Night aims to learn more about light pollution around the world.

Each month features a different campaign and for March the search is on for Canis Major.

Make your observations between the 8th and 17th, and match what you can see to one of eight star charts.

The results are plotted on a world map to track how our view of the dark night sky varies – it now includes more than a decade’s worth of data!

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