Tag: Stargazing

Categories
Nature Stargazing Universe

See the “Bethlehem Star” in the Night Sky

The Great Conjunction of Jupiter and Saturn occurs on 21 December 2020.
The Great Conjunction of Jupiter and Saturn occurs on 21 December 2020.

In astronomy, a conjunction means the “meeting” of two planets in the night sky. Jupiter and Saturn, the biggest planets in our Solar System, can meet as well, but when they do, their conjunction is a “great” one – hence the term Great Conjunction!

Jupiter and Saturn are one of the brightest objects in the night sky. They look just like two very bright stars – but don’t mistake them for stars! They are planets, so they don’t create light themselves just like the stars (like our Sun does for example). They are “lit” only because of the reflection of the sunlight. Which is the same reason why the Moon shines in the night sky!

The Great Conjunction of 2020

In their perpetual movement around the sun, it happens sometimes that planets “meet”. The word “meet” is quoted because the planets don’t actually meet in reality. It is just how we see things from here on Earth. In fact, the two planets are aligned in such a way in their orbits, that from Earth, we see them as if they came into contact with eachother. In reality though, they’re some million kilometres apart!

And why is this conjunction “great” you may ask? Well, this is because Saturn and Jupiter, due to their size, they are already bright even when they’re “apart”. When two bright lights touch each other, they are seen like only one light, even brighter! The same happens with the conjunction of Jupiter and Saturn.

When does it happen?

Jupiter and Saturn are already close in the night sky since quite some time now. But on December 21 (thus tomorrow!), the two planets will be so close (less than 0.1 degrees) that they will look like an elongated, very bright star. So, stargazers, get ready for a Christmas treat!

Why is the Great Conjunction so special this year?

The Great Conjunction occurs regularly, roughly every 20 years. But why is it so special this year?

In function on the position of the “meeting” with respect to the position of the Sun, the conjunction may be more or less bright. If this meeting place is too close to the Sun, the brightness diminishes. This year, the position is ideal for a very bright Great Conjunction!

Another thing to take into account is how close the two planets will appear to be. Not every 20 years the planets have such an apparent closeness! In fact, to get an idea of how rare this occurs, know that last time Jupiter and Saturn appeared so close, was in year 1226! It actually happened in 1623, but it was rather close to the Sun, which made it actually less visible.

Where and how to look?

You need to look in the night sky about 1 hour after sunset, almost anywhere on Earth, even in light polluted areas! In the Northern hemisphere look towards south-west. As for the Southern hemisphere, look towards the west. Needless to say that you will need a clear sky!

It will be impossible to miss such an impressive sight so, don’t worry, it will not be difficult to spot it!

“Christmas Star”

As the Great Conjunction this year happens very close to Christmas, it is also called the “Christmas Star“, or the “Bethlehem Star“. To go even further, some astronomers believe that the Star of Bethlehem which led the Three Wise Men to the place where Jesus was born, might have actually been a Great Conjunction! But there are other theories that suggest that a supernova might have occurred that time. Or, was it indeed a divine miracle?

Categories
Nature Science Stargazing

Twilight Occurrence

Civil twilight in Vadsø
Civil twilight in Vadsø

In a previous article we saw what is twilight and what are the main types of twilight. As a reminder, twilight is the period of the day when a certain point on Earth is illuminated indirectly, by sunlight scattering, when the Sun is below the horizon, but not more than 18°, thus its rays are still visible, indirectly, for an observer located at that certain point.

Now, we’re going to look at how twilight occurs on Earth, when it occurs and… how it doesn’t occur at all in certain places!

Standard twilight occurrence

A standard twilight occurrence was described in our previous article:

During the course of a day, at sunrise, the Sun appears in the sky from the right (East) and, during the whole daytime, it shines its light directly onto the place on Earth where the observer is located. In the evening, at sunset, the Sun will again reach 0° on the left side (West) and it will slowly disappear under the horizon. This moment the evening twilight starts. Due to earth’s rotation, the Sun will continue to descend more and more under the horizon. But before our star reaches 18° under the horizon, there will still be distinguishable light from the Sun for the observer. When the Sun reaches 18°, dusk occurs, and the observer will not distinguish any indirect sunlight anymore and the astronomical night starts.

Because of Earth’s rotation, the Sun continues its trip and, very early in the morning, before it rises, it will reach again 18° under the horizon. At this moment, dawn occurs, and twilight starts again – this time we’re talking about the morning twilight. As time passes more, the Sun will ascend more, until it reaches again 0° and it rises the next day.

Twilight occurs thus during both periods of the day when the Sun is between 0° and 18° under the horizon.

This scenario is true for people living on Earth between approximately 50° North or South of the Equator at any time of the year. This is also valid for higher latitudes, but not around the summer solstice, when the Sun does not descend more than 18° under the horizon during the “night”, and thus there’s no real astronomical night between dusk and dawn.

Continuous twilight between sunset and sunrise

As written in the previous paragraph, above latitudes of approximately 50°N/S, around the date of the summer solstice, the Sun does not descend lower than 18° under the horizon, which means that, even if the Sun is under the horizon, its rays can still be seen, indirectly, during the whole night, which translates itself into a continuous twilight during the whole “night” hours.

In function of the latitude, there can be a continuous astronomical twilight, a continuous nautical twilight, or a continuous civil twilight between sunset and sunrise. This actually occurs in very popular and accessible places around the world, such as:

  • Continuous astronomical twilight: many European countries, such as northern UK, Ireland, the Netherlands, Germany and many other in the Northern Hemisphere or the Falkland Islands in the Southern Hemisphere;
  • Continuous nautical twilight: a great part of Russia and Canada, northern Denmark in the Northern Hemisphere or Ushuaia in Argentina in the Southern Hemisphere;
  • Continuous civil twilight: more northern parts of Russia (such as Sankt Petersburg), Northern Norway, Northern Sweden, Northern Finland.

White nights

A continuous civil twilight between sunset and sunrise is called a white night. The term white night also applies if a certain place does enter nautical twilight also, but if the Sun does not descend lower than 7°.

The white night constitutes a popular symbol for Sankt Petersburg in Russia, where, around the summer solstice, the Sun never goes lower than 7° under the horizon for several days.

A continuous nautical or astronomical twilight does not mean a white night occurs.

No astronomical day between morning and evening twilight

Within the two Polar Circles – Arctic and Antarctic – in wintertime, Polar Night occurs. The polar night means that the Sun does not rise above the horizon at all during 24 hours. But it may approach the horizon, above 18°, thus its rays are seen indirectly and twilight occurs during the “daytime” hours.

Again, in function of the actual latitude, during the normal “daytime” hours, there may be a continuous civil, nautical or astronomical twilight. Vadsø experiences a continuous civil twilight between approximately November 25 and January 17.

Church of Arctic Sea in Vadsø, at noon, during the Polar Night
Church of Arctic Sea in Vadsø, at noon, during the Polar Night’s daytime civil twilight

No twilight at all

In polar regions, around the summer solstice, the Sun is up in the sky 24 hours a day, a period known as the Polar Day. The Sun that never sets for more than 24 hours is called the Midnight Sun, and it never disappears under the horizon for several days in a row. Higher the latitude, longer the period the Midnight Sun occurs.

As the Sun never goes under the horizon, these places experience no twilight at all during all these days.

This condition occurs here in Vadsø during the Midnight Sun period, and lasts more than 2 months, between approximately May 16 and July 26 each year.

Categories
Nature Science Stargazing

Twilight

Me on the shore of the Varanger Fjord, during twilight
Me on the shore of the Varanger Fjord, during twilight

This is not an article about the famous TV show with vampires! In the next few minutes we’ll talk about what the actual twilight period of the day is, when and how it occurs, and what each type of twilight looks like.

What exactly is twilight?

Twilight is the period of the day when a certain point on Earth is illuminated indirectly, by sunlight scattering, when the Sun is below the horizon, but not more than 18°, thus its rays are still visible, indirectly, for an observer located at that certain point. The lower the Sun is below the horizon, the dimmer the twilight. When the Sun reaches 18° below the horizon, the sunrays’ brightness is gone completely and the “real” pitch-black nighttime starts. (Of course, when talking about the Sun being “18° under the horizon”, we mean its “geometrical centre!”)

The image below presents schematically an observer, his line of sight and the position of the (geometrical centre of the) Sun, at various times of the day:

During the course of a day, at sunrise, the Sun appears in the sky from the right (East) and, during the whole daytime, it shines its light directly onto the place on Earth where the observer is located. In the evening, at sunset, the Sun will again reach 0° on the left side (West) and it will slowly disappear under the horizon. This moment the evening twilight starts. Due to earth’s rotation, the Sun will continue to descend more and more under the horizon. But before our star reaches 18° under the horizon, there will still be distinguishable light from the Sun for the observer. When the Sun reaches 18°, dusk occurs, and the observer will not distinguish any indirect sunlight anymore and the astronomical night starts.

Because of Earth’s rotation, the Sun continues its trip and, very early in the morning, before it rises, it will reach again 18° under the horizon. At this moment, dawn occurs, and twilight starts again – this time we’re talking about the morning twilight. As time passes more, the Sun will ascend more, until it reaches again 0° and it rises the next day.

Twilight occurs thus during both periods of the day when the Sun is between 0° and 18° under the horizon.

In function of the position of the Sun below the horizon during twilight, due to the difference of indirect illumination, we distinguish three types of twilight:
● civil twilight – when the Sun is between 0° and 6° under the horizon,
● nautical twilight – when the Sun is between 6° and 12° under the horizon,
● astronomical twilight – when the sun is between 12° and 18° under the horizon.

Civil twilight

Civil twilight at the border of Vadsø Municipality

Evening civil twilight occurs right after sunset and it lasts until the Sun reaches 6° under the horizon. Morning civil twilight occurs before sunrise, when the Sun is 6° and less under the horizon.

During civil twilight, there’s enough indirect light from the Sun so that artificial illumination is not needed. Objects and landscapes are still visible to the unaided eye. This period is especially sought for by photographers, as the lighting creates an amazing effect in pictures. In polar regions, this is when you can experience the beautiful polar blue.

The first bright stars and planets appear in the sky during civil twilight. Venus is usually seen at this time, hence its name “evening star” or “morning star”.

Nautical twilight

Long exposure photo during nautical twilight

Evening nautical twilight occurs when the evening civil twilight ends, thus when the Sun is lower than 6° below the horizon, and it lasts till our star reaches 12° under the horizon. In the morning, nautical twilight occurs when the Sun is 12° below the horizon and until morning civil twilight starts.

The name “nautical twilight” comes from the fact that sailors could still distinguish a visible horizon at sea, and they were still able to navigate thanks to the brightest stars that are perfectly visible in the sky during the nautical dusk. When not at sea, in places where light pollution is absent and when certain atmospheric conditions are met, the unaided human eye may still distinguish shapes or silhouettes of objects.

Long exposure of a nautical twilight scene, on one of Vadsø’s streets

Astronomical twilight

Evening astronomical twilight occurs when the Sun is below 12° relative to the horizon and just before the astronomical night starts – thus when the Sun reaches the 18° point under the horizon. In the morning, astronomical dawn marks the time when the first indirect sunrays appear and until the morning nautical twilight starts.

The end of the evening astronomical twilight marks the moment when the faintest stars and other faint astronomical objects become visible. And, of course, they will stay visible until astronomical dawn. The unaided human eye will generally not be able to detect any light however, and it will consider the sky entirely dark.

***

In the next part, we will look at how twilight occurs in different locations on Earth. Did you know that, here in Vadsø, when the Polar Night starts (around 25 November each year), we experience a continuous civil twilight from around 8 o’clock in the morning and till around 14h? Find out other cool twilight facts here!

Categories
Nature Science Stargazing Universe

The Belt of Venus or the Anti-twilight Arch

Full moon rising underneath the Belt of Venus
Full moon rising underneath the Belt of Venus

You now know why sunsets and sunrises are red (if not, read this article and find out!). But did you know that another beautiful light phenomenon occurs at sunset and sunrise, besides the beautiful reddish skies? It actually occurs opposite of where the Sun is setting or rising, thus opposite of where all the beautiful reddish colors light up the sky! So, next time you have a clear sky, try looking away from the nice sunset, in order to see the anti-twilight arch! But what is this anti-twilight arch?

Let’s explain its name first

The Belt of Venus, which is a stylized name for the anti-twilight arch, is a pink glow above the horizon, right opposite of where the Sun sets/rises. This opposite place of the Sun is actually an imaginary point, which we will call from now on the antisolar point (anti means opposite).

The phenomenon takes place during twilight – thus before sunrise or after sunset respectively. It is represented by a pink glow that surrounds, just like an arch, the horizon opposite of where sunsets and sunrises occur.

So there you have it – the anti-twilight arch!

Concerning the name “Belt of Venus”, contrary of what you might guess, it’s got absolutely nothing to do with the planet Venus, or any of its belts or rings (…which do not exist anyway!). Planet Venus has a smaller orbit around the Sun than Earth does, and this makes Venus visible to our eyes only around sunsets and sunrises, similar to how the antitwilight arch becomes visible at sunset and sunrise. This is the only association that the Belt of Venus might have with the actual planet. The name Belt of Venus is, in fact, inspired from the girdle which was supposedly worn by the goddess Venus and which might resemble the pinkish arch around the antisolar point, at twilight.

So what exactly is this Belt of Venus?

After sunset (or before sunrise), the Sun is below the horizon, relative to an observer on Earth. In the figure below, the observer’s line of sight is represented by the thin grey line and the Sun is below this line of sight, thus below the observer’s horizon. The dotted circle around Earth represents our planet’s atmosphere. Even though the Sun is below the horizon, right after sunset, light rays from the Sun still make way to get to the observer and even further (red arrow), till above the antisolar point, where they get backscattered off Earth’s atmosphere (pink arrow). This region, where the backscattering takes place, has a belt shape, and this belt is nothing else but the antitwilight arch!

In this figure, notice that the backscattering “point”, at the tip of the red arrow, is a little above the line of sight for the observer. The small region between the backscattering point and the line of sight, is represented by a dark belt, which is our planet’s shadow.

What will you, as observer, see? Well, if you look right opposite where the Sun is setting, you will notice, right after sunset, a faint pinkish light, stretching around the antisolar point, like a belt, or arch. As time passes, this pinkish glow will rise. Right underneath it, you will see a darker belt, which is nothing else than Earth’s shadow! As time passes further, the pinkish glow will rise even more, as will our planet’s shadow, until night will take over entirely and it will become pitch black outside.

This effect is sometimes very faint, and in order to get a good view, you will need, first of all, a clear sky. Best is also to have a clear horizon above the antisolar point as well, in order to distinguish this effect as better as possible.

So, now that you know about the antitwilight arch, I dare you to ignore a beautiful sunset and look right opposite! But I promise that if you do, you will get to see another magnificent optical phenomenon, less known, but of equal beauty! Have you already seen the Belt of Venus?

The Belt of Venus. Image credit: Kent Duryee (https://commons.wikimedia.org/wiki/File:Belt-of-venus.jpg), „Belt-of-venus“, https://creativecommons.org/licenses/by/4.0/legalcode
The Belt of Venus.
Image credit: Kent Duryee (https://commons.wikimedia.org/wiki/File:Belt-of-venus.jpg), „Belt-of-venus“, https://creativecommons.org/licenses/by/4.0/legalcode
Categories
Nature Stargazing Universe

Comet Neowise

Comet Neowise
Comet Neowise

You have probably heard or read in the news recently about a magnificent sight in the night sky in the Northern hemisphere – the Neowise comet. It’s been discovered since March 2020 with the help of the Neowise space telescope, hence its name, but at the time of discovery it was of a +10 magnitude, thus not visible to the naked eye.

This month, however, its magnitude reduced to around +3 – which means that the comet is not only now visible to the naked eye, but it can also be seen in relatively light polluted areas, such as bigger cities! It is actually so bright that it is the second brightest comet in the night sky after the Hale-Bopp comet of 1997. Remember it?

So, if you live in Earth’s northern hemisphere, and if your sky is clear of clouds (…and, of course, if you don’t live in polar regions, where the Polar Day currently occurs), do take a look at the night sky! Best is to look just after the nautical twilight, or when the stars start being seen in the North-north-west direction, just a little bit above the horizon! More exactly, locate the well known asterism Big Dipper and look just right under it! You will not regret it!

And if you want to take photos of the comet, don’t forget your tripods! I forgot it and the photos look blurry, but still nice! Did you manage to take nice pictures of it? And remember, if you’d like to get to know the night sky better – do visit Vadsø with me next winter, where the perspective of the zenith is completely different and where light pollution almost doesn’t exist, and let’s go on an Arctic Stargazing adventure together!

Photo of Comet Neowise taken with my phone and without a tripod!