Looking Forward – And Up – For 2017!

There are a lot of exciting things happening in 2017. Many are covered in detail on large astronomy web sites like Sea and Sky: http://www.seasky.org/astronomy/astronomy-calendar-2017.html

Here are just a few highlights to consider.

11 February 2017 – Lunar Eclipse

Following on from the full moon earlier on the same day, the moon will pass into the edge of the Earth’s shadow for a “penumbral lunar eclipse.” We should be in a great position to see the moon darkening in Ontario.  Here’s a link to a NASA PDF on the event: https://eclipse.gsfc.nasa.gov/LEplot/LEplot2001/LE2017Feb11N.pdf

1 April 2017 – Mercury at Greatest Eastern Elongation

The tiny planet Mercury will be visible in the evening sky in early spring; on 1 April it reaches its greatest eastern elongation, and will be visible in the evening sky at sunset.

7 April 2017 – Jupiter at Opposition

On 7 April the Earth will pass directly between Jupiter and the sun. The planet will be very bright in the night sky, rising at sunset. Even a small telescope should reveal the four Galilean moons of our solar system’s largest planet.

15 June 2017 – Saturn at Opposition

In mid-June Saturn and its magnificent rings will be as bright as possible this year. Like Jupiter in April, at opposition the Earth lies directly between Saturn and the sun. Rising at sunset, the planet will appear as a fully-illuminated disk through a modest telescope, nestled within its amazing rings.

cassini_saturn_orbit_insertion

Saturn will be worth watching in 2017 on another front. The Cassini mission is drawing to a close. Throughout the year, NASA mission controllers are swinging the wonderful car-sized spacecraft through Saturn’s rings for the first time, willing to take risks at the tail end of the voyage. Launched 20 years ago (1997), Cassini reached Saturn in 2004 and has been performing nearly flawlessly ever since. Later in 2017 the mission will be brought to an end and the spacecraft will be plunged into Saturn itself, a fiery demise to ensure that the environments of Titan and the other moons of Saturn are not contaminated. The feature image on this post is an artist’s rendering of Cassini and its attached Huygens probe undergoing the orbital insertion maneuver over Saturn in 2004 (Public Domain image; source NASA: http://photojournal.jpl.nasa.gov/catalog/PIA03883).

21 August 2017 – The Great Eclipse

Perhaps one of the big events in 2017 will be the “Great Eclipse” – a total solar eclipse that will cross the continental United States from west to east coasts. On Monday 21 August 2017 the moon will pass directly between the Earth and the Sun, casting a vast circular shadow and giving millions of people a chance to see a true natural spectacle. Totality will pass through states like Kentucky and Tennessee, but from Ontario we will still see a great partial eclipse in the afternoon. Here’s NASA’s posting for eclipse information: http://eclipse.gsfc.nasa.gov/SEplot/SEplot2001/SE2017Aug21T.GIF

13 November 2017 – Close Conjunction of Venus and Jupiter

Just before sunrise Venus and Jupiter will be very close to each other in the sky – just 0.3 degrees apart, or less than the diameter of the full moon.

I hope you can get out and enjoy these and other exciting sky events in 2017! As we get closer to each I will post additional information on viewing – and when possible taking pictures of – these events.

 

Advertisements

The Equinox is coming. So is the Equilux. So what’s the difference? About four days.

A term from astronomy that has entered common use is Equinox, which translated from its Latin roots roughly means “Equal Night.” Most people think that the Spring (or Vernal) Equinox and the Fall (or Autumnal) Equinox are the days when the length of the daylight and the length of the night are equal. Close, but not quite right.

As the days lengthen from winter toward summer, there certainly is a day during which the length of time we see sunlight and the length of time it’s dark are roughly the same. Technically, however, the timing of each Equinox is defined as the moment when the earth passes a particular point in its orbit around the sun. It’s not defined by the local length of the day.

We have extended our imaginations into space, and covered it with geometric patterns. One of those patterns is an imaginary circle that runs around the entire sky and is called the ecliptic. This is the circle that marks the apparent position of the sun throughout each year, relative to the background stars. It’s called “ecliptic” because of its importance in determining the dates of eclipses. For a lunar or a solar eclipse to take place, the sun, moon, and earth all have to be in alignment with the ecliptic.

Because much of the solar system is in a pretty flat configuration, most of what happens in the solar system also happens near the ecliptic. The orbits of the earth, Mars, Venus, Jupiter, and many other things in the solar system lie in more or less the same plane. So, when you are able to spot any of the planets you’re seeing, approximately, where the ecliptic lies in the sky.

Another similar geometric pattern or circle that is projected into the sky for practical reasons is defined by the earth’s own equator. Astronomers can plot a line across the sky which corresponds, in the “up” direction, to where the equator is on earth. If that’s a little hard to envision, just consider the North Star, Polaris. By accident it sits within a degree of the place in the sky directly above the earth’s north pole (the rotational pole, not the magnetic pole. They’re different things, for another day). From Polaris the celestial equator is 90 degrees south in all directions.

Right. So, two circles projected up in the sky (Picture an orange with two rubber bands around its middle that cross each other as viewed from the inside). They are tilted with respect to each other by the same amount as the earth’s rotation is tilted relative to the position of the sun – around 23 degrees. Each Equinox is defined as the time at which the earth passes the place where the two lines cross. At that moment, the terminator – the place where the sunlit side of the earth and the night side of the earth meet – is at right angles to the earth’s equator.

Now, you’d think that this perpendicular arrangement would mean day and night are of equal length, and geometrically it’s true. However, what we actually see is much more complicated. The apparent length of daylight and night differs from place to place on earth. One of the big factors is the bending effect of the earth’s atmosphere. We sit under an ocean of air, and one of the consequences of our atmosphere is that light is bent as it comes in from space; air acts like a huge lens. The closer an object is to the horizon (like the sun at sunset or sunrise) the more the bending is apparent. This is because the light at these low angles must pass through much more air to reach us. It acts as a thicker lens closer to the horizon.

The consequence of this bending of sunlight is that the time something actually happens in the sky is not necessarily the time we see it happening from our vantage point under the atmosphere. In fact, this bending is enough that at the horizon it’s enough to make the sun appear to be more than its own diameter “earlier” in rising than it would without the atmosphere there. A staggering thought – when we see the sun just getting up over the horizon in the morning, it’s actually still below the horizon geometrically!

The other factor is the apparent size of the sun. From the earth’s perspective the sun is about a half a degree across, compared to the whole 360 degrees of the sky. Sunrise and sunset do not happen instantaneously. Sunrise is defined as the moment that the sun’s disk just appears on the horizon for any particular morning. Sunset is defined as the moment when the last bit of the sun just disappears past the horizon from any particular location.

These two things combined are enough of an effect that the Equilux, the day that has equal hours and minutes of sun above and below the horizon, is about four days before the Vernal Equinox and about four days after the Autumnal Equinox in the area of Hamilton, Ontario. The timing of the Equilux may be different in your location. In 2013 the Spring Equilux – the date that daylight and nighttime hours are closest to being 12 and 12 – falls on 16 March. The corresponding Equinox this year takes place about 7:02 AM on 20 March.

In September 2013 the Autumnal Equinox, marking the official beginning of fall, is at 4:44 PM on the 22nd. The Fall Equilux, however, for Hamilton is on 25 September.

Sources:

  • Thanks to the on-line calendar of the Hamilton Amateur Astronomers for the Equilux and Equinox dates and times for 2013. http://amateurastronomy.org/
Copyright © David Allan Galbraith 2013