Eye on the Night Sky over Whitby

An artist's illustration incorporating new Hubble observations of aurora at Jupiter.
An artist's illustration incorporating new Hubble observations of aurora at Jupiter.

Planetary Skylights

Venus, the brilliant evening star, dominates the western twilight sky.

Indeed it seems unusual for Venus to be above the horizon well after midnight, not setting until around 1am.

Venus is keeping pace with the sun, moving through Gemini during May and drawing ever closer to Jupiter, the other very conspicuous planet.

By the end of May Venus is in line with the ‘Twins’ lead stars; Castor and Pollux. Through a scope Venus reveals very little except a phase, which is around half.

A crescent moon lies nearby on May 21st, a great twilight photo opportunity.

For the first three weeks of May Mercury continues its spring evening apparition low in the WNW sky.

Look for it 45 minutes after sunset a ‘fist’ height above the horizon, lower right of Venus.

Use binoculars if you cannot initially spot it with just the naked eye.

The moon passes in the vicinity on the 19th, although Mercury will be more difficult to spot by then.

Jupiter is well-placed quite high in the SW aspect of the sky, but is gradually slipping down westwards towards the horizon. As it does so, Venus draws ever closer.

The pair will have a spectacular conjunction later in June. The moon lies nearby on the 24th.

Finally, and certainly not least, Saturn comes to opposition on May 23, residing due south at midnight.

Before then you will be able to view the ‘ringed wonder’ from 10pm across in the east, a steady bright ‘pearly star’ residing above the ruddy hue of Antares, chief star in Scorpius.

Through a scope the ring system appears wide open and is a magnificent sight.

Look for Titan, Saturn’s largest moon as a speck of light nearby.

Our moon lies nearby on the 6th.

Meteor activity The Eta Aquarid meteor shower peaks in the early morning hours on May 6, as Aquarius is rising.

This is one of two meteor showers associated with debris particles deposited over time by Comet Halley, the other shower being the Orionids, which peak in late October.

Although the Eta Aquarids have a ZHR (zenith hourly rate) approaching 30, Aquarius is only just rising in the SE shortly before dawn and actual observed rates will be less than half of this at best.

A waning gibbous moon will also restrict numbers.

In Focus - Twilight and Shadow

As nights become ever lighter, now may be a good opportunity to briefly explain why the changes in light levels encountered daily after sunset or before sunrise occur.

If you were to step outside in autumn, winter or early spring, skies are fully darkened by around 9pm.

This deepest level of darkness, known as Astronomical twilight, is just one of three levels experienced, the others being Civil and Nautical twilight respectively.

However, because of our location (latitude) on the surface of the globe, as Earth orbits around the Sun, the duration of twilight levels varies considerably.

So let me explain a little further using evening twilight as an example.

Immediately after sunset Civil twilight exists until the Sun is approximately 6 degrees below the horizon.

At this point normal daytime activities that require light cease to be possible and ‘lighting up’ time occurs. If skies are really clear look out for an interesting phenomena half an hour or so after sunset, opposite the position of sunset.

In this direction, a hazy, dark-ish purple band just above the ESE horizon gradually ‘heaps up’ before being engulfed by deepening twilight.

This is not mist but the shadow of Earth itself, being cast back into space.

As the Earth spins a little further round and the Sun’s elevation below the horizon reaches approximately 12 degrees, the marine horizon is no longer apparent and Nautical twilight exists.

In the heavens only the brighter navigational stars are visible.

Finally, when the Sun drops approximately 18 degrees below the horizon (a hands span) Astronomical twilight commences and the remaining fainter stars become visible, unless light pollution drowns them out.

With the approach of dawn the same lighting levels are experienced - only in reverse.

As we move toward summer the northern hemisphere is inclined towards the Sun and twilight duration’s alter significantly. From the latitude of Whitby astronomical twilight is totally absent for 82 nights; from late May until mid August.

The Sun’s journey beneath the horizon is so shallow it never actually reaches 18 degrees.

Matters become worse (for the astronomer) around the summer solstice when even nautical twilight levels barely exist here, the Sun setting by a mere dozen degrees.

At least from northern England summer astronomy is just about possible, but as you travel further north nocturnal conditions grow ever lighter and only 13 degrees north of Whitby at latitude 67 degrees N (the Arctic Circle) twilight on any level is completely absent for part of summer and the Sun is visible above the horizon 24 hours a day.

Only solar observers are completely happy for in the land of “midnight sun” star gazing is limited to just one, that is if it’s not cloudy!

For more information please contact Mark Dawson on (01947) 605516 or e-mail spanton33@talktalk.net.

Visit www.whitby-astronomers.com

The final Whitby and District Astronomical Society monthly meeting before summer recess is tonight (Tuesday May 5) at Caedmon College, room H1, 7.30pm.