The focus of this week’s class was Celestial Rhythms and the development of the constellations.
We started the class upstairs in the observatory and outside on the balcony with Paul describing our session and then identifying a few of the objects on display in the night sky, including a crescent Moon. Paul described how the Moon is moving around the Earth and how the position of the Sun in relation to the Earth and Moon affects the different phases of the Moon. He explained that next week the moon will have moved across the sky in counter-clockwise direction and showed us approximately where it would be.
We then headed downstairs and outside to a large model of the solar system painted on the ground, as seen in the image below during the daytime. We sat around the model and Paul described the motion of the Earth and other planets around the Sun. This demonstration was a great way to visualise our solar system and how various planets move in relation to the Sun. Paul also described how our view on Earth is affected by the Sun and how the Sun blocks various stars at certain parts of the year.
This session was about the importance of understanding how the sky moves during a night and throughout a whole year.
“The sky is on a continual march that presents us with different constellations at different times of night, and different times of year.”
After our outside demonstration and further observing of the night sky we moved back inside for a much needed tea break and to escape the chilly Sydney evening weather, should have taken a jacket!
After our break we headed downstairs to the 3D theatre and enjoyed another presentation by Paul. The focus of the inside session was to demonstrate the motion of the Earth travelling around the Sun and its rotation on its own axis, what causes the seasons as well as examine closely what makes a day and a year, which is not quite what we thought.
I learnt so much over our 2 and a half hour class. As we spin around our axis once per day we actually move about 1 degree around the Sun. We rotate and orbit in a counterclockwise direction and in a year there are 365.25 days. So, the Earth, rotating at 1360 km/h, must spin on its own axis 360 + 1 degrees to have the Sun reappear in the same position. A day as we know it is 24 hours long, this is actually a solar day as our clocks have been tuned to the combined motion of our rotation and orbit around the Sun.
A different kind of day is called a sidereal day, which is 23 hours 56 minutes and 4 seconds long. This is the time astronomers use to predict the location of stars on the night sky. All of this means that we will see a slightly different sky each night.
We all know we have four seasons on Earth and the seasons are caused by our axis of rotation being tilted by 23.5 degrees to the plane of our orbit, called the ecliptic. The axis points in the same direction as we orbit the Sun, and this is what causes the seasons and the variation in the length of a day throughout the year. Paul gave us examples of the equinoxes and the solstices. The equinoxes occur approx. half way between the solstices on March 21 and 21 September, the solstices on 21 June and 21 December. In June the axis is pointed towards the Sun so the northern hemisphere will see the Sun high in the sky and will receive many hours of sunbathed daylight. In the southern hemisphere people will see the Sun much lower in the sky and will experience less daylight compared to the northern hemisphere. The reverse is true for December. At equinox the day and night are the same length for both hemispheres – equal.
“We spend our days and nights humbly on planet Earth. We suffer from the illusion that we feel stationary on the surface of the Earth and that the Sun and stars seem to revolve around us.”
Infant, our axis is slowly precessing, like a spinning top over a period of 26,000 years. This means that the celestial poles on the celestial sphere sweep out a circle every 26,000 years carrying our coordinate system of the stars with it. This small drift was actually measured by Hipparchus around 140 BC. Paul has recommended we purchase a planisphere, seen below, a dynamic map that can portray the night sky for a given location at any time of night throughout the year. These are only designed for one latitude and do not contain much information.
There are 88 constellations in the night sky. Each constellation is an area of the sky and are recognised and classified by the International Astronomy Union (IAU). We learnt heaps about the constellations:
- Each culture developed its own constellations
- Constellations often symbolised mythological creatures
- Constellations are a reminder of a lesson or a seasonal event
- Many originated in Babylonia
- The Greeks adopted many and developed their own mythologies to explain them
- The constellations the Sun passes through along the ecliptic are called the zodiacs
- There are actually 13 zodiacs, including Ophiuchus, although the Sun does not spend too much time in this sign
- The Babylonians set up the zodiacs and their counting system was based on base 12, so they stuck with just the 12 constellations and that is why we have 12 hours of day, 12 hours of night and 12 months a year
- The Sun now spends more time in Ophiuchus than in some neighbouring constellations
I am a Pisces and here is my constellation from the IAU (http://www.iau.org/static/public/constellations/gif/PSC.gif)
A few facts about Pisces:
- One of the most ancient constellations
- Depicts 2 fishes swimming in opposite directions with their tails joined
- In mythology the two fishes represent Aphrodite (Venus) and Eros (Cupid)
- One day they hide to hide in rushes along the Euphrates to escape a monster called Typhon. The two fishes swam away to safety
- Pisces is watery and faint, it lies in an undistinguished part of the sky
- Its brightest stars are only magnitude 4
- One way to locate Pisces is by reference to the square of Pegasus
- The Sun passes through Pisces between Feb 19 and March 20
- The 12th sign
- Famous Pisces include Steph Curry (Mar 14), Einstein (Mar 14), Steve Jobs (Feb 24), Daniel Craig (Mar 2), Jon Hamm (Mar 10)
Eta Piscium is the brightest star at 316x that of the Sun. It is 294 light years from Earth and is a G class bright giant star.
Pisces contains a Messier object called Messier 74, a spiral galaxy located between the stars alpha Arietis and eta Piscium. It is also known as the Phantom Galaxy, shown below.
The grand-design spiral galaxy Messier 74 as photographed by the Hubble Space Telescope in 2007. Image: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: R. Chandar (University of Toledo) and J. Miller (University of Michigan)
The red areas indicate pockets of hydrogen gas. They glow due to the radiation from hot, young stars. Astronomers call these areas H2 regions. The brighter stars are not part of the galaxy and are actually located a lot nearer to us. The galaxy appears face-on and is approx. 30 million light years from Earth. It is roughly the same size as the Milky Way with a diameter of 95,000 light years. Two supernovae have been seen exploding in recent years in this galaxy. It contains about 100 billion stars. It is not easy to observe due to low surface brightness and requires clear skies. The only other object with a lower surface brightness is M101, the Pinwheel Galaxy, shown below.
The Phantom is an example of a grand design spiral galaxy with 2 clearly defined arms which extend for about 1,000 light years. The arms contain clusters of young blue stars and star forming nebulae. It is receding at a speed of 793 km/s.
Zoom into M74:
M74 was first observed in 1780 by French astronomer Pierre Méchain who told his good friend Charles Messier about it, Charles added it to his famous catalogue.
More about M74 here.
Thank you Hubble!