Our Sun undergoes regular cycles of activity that last eleven years, on average; however, the variation in the output of solar radiation is slight — only about 0.1%. Though we are currently approaching another solar maximum, expected sometime next year, the increase in solar activity does not account for the early arrival of spring-like weather. It does, however, have other effects: It produces more sunspots and solar flares, and it heralds the return of beautiful auroral displays.
Auroras occur when charged electrons from the solar wind interact with the rarefied gases in the upper atmosphere, along the Earth’s north and south auroral ovals, centred on the magnetic poles. These charged particles stream away from the Sun at speeds approaching 750 km/sec: About two days later, after arriving at Earth, they follow our planet’s geo-magnetic field lines, down through the upper atmosphere, as they converge at both magnetic poles. At altitudes of between 50 to 300 kilometres, these solar electrons excite atoms of oxygen and nitrogen, causing the air to glow just like a neon light. The colour of the auroras depends on the type of atom that’s being excited — and its altitude: Oxygen produces both green and red at altitudes of up to 250 km; nitrogen emits blue and purple at lower altitudes of around 100 km.
According to Cree and Inuit legend, the Northern Lights represent the spirits of the departed, dancing in the northern sky. However, today we know this ethereal dance is really produced by the interaction of magnetic and electrical forces, reacting with one another in constantly changing patterns, high in the Earth’s atmosphere.
There are also many stories about hissing or crackling sounds associated with auroras, but no audio recordings of auroral sounds have ever been made. In fact, most scientists are sceptical that the Northern Lights can be “heard” for two reasons: At altitudes of 50 to 300 kilometres the air is far too thin to conduct sound; and at those distances, there would be a time-lag between what is seen and what is heard. However, a few researches are currently investigating the idea that auroral sounds are literally produced “in your head” — that the brain re-interprets part of the visual stimulus as sound. One way to test this hypothesis is to close your eyes, if ever you “hear” auroras, and notice whether the sound disappears.
And now, onto the planets…
Venus and Jupiter at twilight
This month, Venus and Jupiter illuminate the western sky at twilight, and since they are the two brightest planets, they are impossible to miss. Venus is simply radiant as the Evening Star: On April 3, the dazzling planet is located just half-a-degree from the Pleiades star cluster, in Taurus. Meanwhile, Jupiter shines half as bright and appears closer to the western horizon. The situation remains constant throughout the month, with Venus gaining altitude and Jupiter sinking lower in the sky. Toward mid-month, Venus sets just before midnight — more than four hours after the Sun! As for Jupiter, the giant planet continues its descent and disappears in the glow of sunset at month’s end.
An extremely thin, 1 ½ day-old crescent Moon will appear just below Jupiter on the evening of April 22: Scan the western sky with binoculars and you are sure to find the duo in the glow of twilight. Two days later, on April 24, the waxing lunar crescent will appear below Venus: Look for the soft glow of earthshine illuminating the otherwise dark portion of the lunar surface. This combination of the Moon and Venus at twilight is one of the most beautiful sights in the sky!
Mars sets before dawn
Mars is already high in the southeast by nightfall, shining brightly among the stars of Leo, just to the left of Regulus. The Red Planet, which is certainly distinguishable by its tell-tale colour, has been moving in retrograde since January 25; however, that ends on the 15th of this month, when Mars resumes its direct, eastward movement and begins gaining distance from Regulus once more. Look for the waxing gibbous Moon to appear below Mars on the evenings of April 3 & April 30.
Saturn all night long
This is the best month to observe Saturn: On April 15, the ringed planet is in opposition, which means is rises in the east at sunset, culminates high in the south around midnight, and sets in the west at sunrise. Saturn appears right next to Spica, the brilliant blue star in Virgo, where it will remain until it disappears in the glow of sunset this fall. Its rings are currently tilted about 14 degrees, which makes Saturn a prime object for small amateur telescopes. To help you identify the ringed planet, the full Moon will appear near Saturn, and Spica, on the night of April 6 to 7.