Occultation Palooza: The Moon Covers Aldebaran and More

Aldebaran

This week, we thought we’d try an experiment for tonight’s occultation of Aldebaran by the Moon. As mentioned, we’re expanding the yearly guide for astronomical events for the year in 2017. We’ve done this guide in various iterations since 2009, starting on Astroguyz and then over to Universe Today, and it has grown from a simple Top 10 list, to a full scale preview of what’s on tap for the following year.

You, the reader, have made this guide grow over the years, as we incorporate feedback we’ve received.

Anyhow, we thought we’d lay out this week’s main astro-event in a fashion similar to what we have planned for the guide: each of the top 101 events will have a one page entry (two pages for the top 10 events) with a related graphic, fun facts, etc.

So in guide format, tonight’s occultation of Aldebaran would break down like this:

Wednesday, September 21st: The Moon Occults Aldebaran

The occultation footprint of tonight's Aldebaran event.
The occultation footprint of tonight’s Aldebaran event.

Image credit Occult 4.2

The 67% illuminated waning gibbous Moon occults the +0.9 magnitude star Aldebaran. The Moon is two days prior to Last Quarter phase during the event. Both are located 109 degrees west of the Sun at the time of the event. The central time of conjunction is 22:37 Universal Time (UT). The event occurs during the daylight hours over southeast Asia, China, Japan and the northern Philippines and under darkness for India, Pakistan and the Arabian peninsula and the Horn of Africa. The Moon will next occult Aldebaran on October 19th. This is occultation 23 in the current series of 49 running from January 29th 2015 to September 3rd, 2018. This is one of the more central occultations of Aldebaran by the Moon for 2016.

india-view

The view from India tonight, just before the occultation begins. Image credit: Stellarium

Fun Fact-In the current century, (2001-2100 AD) the Moon occults Aldebaran 247 times, topped only by Antares (386 times) and barely beating out Spica (220 times).

Or maybe, another fun fact could be: A frequent setting for science fiction sagas, Aldebaran is now also often confused in popular culture with Alderaan, Princess Leia’s late homeworld from the Star Wars saga.

Like it? Thoughts, suggestions, complaints?

Now for the Wow! Factor for tonight’s occultation. Aldebaran is 65 light years distant, meaning the light we’re seeing left the star in 1951 before getting photobombed by the Moon just over one second before reaching the Earth.

There are also lots of other occultations of fainter stars worldwide over the next 24 hours, as the Moon crosses the Hyades.

And follow that Moon, as a series of 20 occultations of the bright star Regulus during every lunation begins later this year on December 18th.

Gadi Eidelheit managed to catch the March 14th, 2016 daytime occultation of Aldebaran from Israel:

And also in the ‘Moon passing in front of things’ department, here’s a noble attempt at capturing a difficult occultation of Neptune by the Moon last week on September 15th, courtesy of Veijo Timonen based in Hämeenlinna Finland:

Lets see, that’s a +8th magnitude planet next to a brilliant -13th magnitude Moon, one million (15 magnitudes) times brighter… it’s amazing you can see Neptune at all!

Last item: tomorrow marks the September (southward) equinox, ushering in the start of astronomical fall in the northern hemisphere, and the beginning of Spring in the southern. The precise minute of equinoctial crossing is 14:21 UT. In the 21st century, the September equinox can fall anywhere from September 21st to September 23rd. Bob King has a great recent write-up on the equinox and the Moon.

Here's EVERY occultation of Aldebaran from 2015 through 2018. (Click to enlarge) Credit: Occult 4.2.
Here’s EVERY occultation of Aldebaran from 2015 through 2018. (Click to enlarge) Credit: Occult 4.2.

Don’t miss tonight’s passage of Aldebaran through the Hyades, and there’s lots more where that came from headed into 2017!

The Orbit of Earth. How Long is a Year on Earth?

Ever since the 16th century when Nicolaus Copernicus demonstrated that the Earth revolved around in the Sun, scientists have worked tirelessly to understand the relationship in mathematical terms. If this bright celestial body – upon which depends the seasons, the diurnal cycle, and all life on Earth – does not revolve around us, then what exactly is the nature of our orbit around it?

For several centuries, astronomers have applied the scientific method to answer this question, and have determined that the Earth’s orbit around the Sun has many fascinating characteristics. And what they have found has helped us to understanding why we measure time the way we do.

Orbital Characteristics:

First of all, the speed of the Earth’s orbit around the Sun is 108,000 km/h, which means that our planet travels 940 million km during a single orbit. The Earth completes one orbit every 365.242199 mean solar days, a fact which goes a long way towards explaining why need an extra calendar day every four years (aka. during a leap year).

The planet’s distance from the Sun varies as it orbits. In fact, the Earth is never the same distance from the Sun from day to day. When the Earth is closest to the Sun, it is said to be at perihelion. This occurs around January 3rd each year, when the Earth is at a distance of about 147,098,074 km.

The average distance of the Earth from the Sun is about 149.6 million km, which is also referred to as one astronomical unit (AU). When it is at its farthest distance from the Sun, Earth is said to be at aphelion – which happens around July 4th where the Earth reaches a distance of about 152,097,701 km.

And those of you in the northern hemisphere will notice that “warm” or “cold” weather does not coincide with how close the Earth is to the Sun. That is determined by axial tilt (see below).

Elliptical Orbit:

Next, there is the nature of the Earth’s orbit. Rather than being a perfect circle, the Earth moves around the Sun in an extended circular or oval pattern. This is what is known as an “elliptical” orbit. This orbital pattern was first described by Johannes Kepler, a German mathematician and astronomer, in his seminal work Astronomia nova (New Astronomy).

An illustration of Kepler's three laws of motion, which show two planets that have elliptical orbits around the Sun. Credit: Wikipedia/Hankwang
An illustration of Kepler’s three laws of motion, which show two planets that have elliptical orbits around the Sun. Credit: Wikipedia/Hankwang

After measuring the orbits of the Earth and Mars, he noticed that at times, the orbits of both planets appeared to be speeding up or slowing down. This coincided directly with the planets’ aphelion and perihelion, meaning that the planets’ distance from the Sun bore a direct relationship to the speed of their orbits. It also meant that both Earth and Mars did not orbit the Sun in perfectly circular patterns.

In describing the nature of elliptical orbits, scientists use a factor known as “eccentricity”, which is expressed in the form of a number between zero and one. If a planet’s eccentricity is close to zero, then the ellipse is nearly a circle. If it is close to one, the ellipse is long and slender.

Earth’s orbit has an eccentricity of less than 0.02, which means that it is very close to being circular. That is why the difference between the Earth’s distance from the Sun at perihelion and aphelion is very little – less than 5 million km.

Seasonal Change:

Third, there is the role Earth’s orbit plays in the seasons, which we referred to above. The four seasons are determined by the fact that the Earth is tilted 23.4° on its vertical axis, which is referred to as “axial tilt.” This quirk in our orbit determines the solstices – the point in the orbit of maximum axial tilt toward or away from the Sun – and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular.

Over the course of a year the orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation. These changes in the pattern of radiation reaching earth’s surface cause the succession of the seasons. Credit: NOAA/Thomas G. Andrews
Over the course of a year the orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation. Credit: NOAA/Thomas G. Andrews

In short, when the northern hemisphere is tilted away from the Sun, it experiences winter while the southern hemisphere experiences summer. Six months later, when the northern hemisphere is tilted towards the Sun, the seasonal order is reversed.

In the northern hemisphere, winter solstice occurs around December 21st, summer solstice is near June 21st, spring equinox is around March 20th and autumnal equinox is about September 23rd. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

While it is true that Earth does have a perihelion, or point at which it is closest to the sun, and an aphelion, its farthest point from the Sun, the difference between these distances is too minimal to have any significant impact on the Earth’s seasons and climate.

Lagrange Points:

Another interesting characteristic of the Earth’s orbit around the Sun has to do with Lagrange Points. These are the five positions in Earth’s orbital configuration around the Sun where where the combined gravitational pull of the Earth and the Sun provides precisely the centripetal force required to orbit with them.

Sun Earth Lagrange Points. Credit: Xander89/Wikimedia Commons
Sun-Earth Lagrange Points. Credit: Xander89/Wikimedia Commons

The five Lagrange Points between the Earth are labelled (somewhat unimaginatively) L1 to L5. L1, L2, and L3 sit along a straight line that goes through the Earth and Sun. L1 sits between them, L3 is on the opposite side of the Sun from the Earth, and L2 is on the opposite side of the Earth from L1. These three Lagrange points are unstable,  which means that a satellite placed at any one of them will move off course if disturbed in the slightest.

The L4 and L5 points lie at the tips of the two equilateral triangles where the Sun and Earth constitute the two lower points. These points liem along along Earth’s orbit, with L4 60° behind it and L5 60° ahead.  These two Lagrange Points are stable, hence why they are popular destinations for satellites and space telescopes.

The study of Earth’s orbit around the Sun has taught scientists much about other planets as well. Knowing where a planet sits in relation to its parent star, its orbital period, its axial tilt, and a host of other factors are all central to determining whether or not life may exist on one, and whether or not human beings could one day live there.

We have written many interesting articles about the Earth’s orbit here at Universe Today. Here’s 10 Interesting Facts About Earth, How Far is Earth from the Sun?, What is the Rotation of the Earth?, Why are there Seasons?, and What is Earth’s Axial Tilt?

For more information, check out this article on NASA- Window’s to the Universe article on elliptical orbits or check out NASA’s Earth: Overview.

Astronomy Cast also espidoes that are relevant to the subject. Here’s BQuestions Show: Black black holes, Unbalancing the Earth, and Space Pollution.

Sources:

Precession of the Equinoxes

Semi Major Axis

When he was first compiling his famous star catalogue in the year 129 BCE the Greek astronomer Hipparchus noticed that the positions of the stars did not match up with the Babylonian measurements that he was consulting. According to these Chaldean records, the stars had shifted in a rather systematic way, which indicated to Hipparchus that it was not the stars themselves that had moved but the frame of reference – i.e. the Earth itself.

Such a motion is called precession and consists of a cyclic wobbling in the orientation of Earth’s axis of rotation. Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative, and takes 25,772 years for a full precession to occur. This has historically been referred to as the Precession of the Equinoxes.

The name arises from the fact that during a precession, the equinoxes could be seen moving westward along the ecliptic relative to the stars that were believed to be “fixed” in place – that is, motionless from the perspective of astronomers – and opposite to the motion of the Sun along the ecliptic.

This precession is often referred to as a Platonic Year in astrological circles because of Plato’s recorded remark in the dialogue of Timaeus that a perfect year could be defined as the return of the celestial bodies (planets) and the fixed stars to their original positions in the night sky. However, it was Hipparchus who is first credited with observing this phenomenon, according to Greek astronomer Ptolemy whose own work was in part attributed to him.

The precession of the Earth’s axis has a number of noticeable effects. First of all , the positions of the south and north celestial poles appear to move in circles against the backdrop of stars, completing one cycle every 25, 772 years. Thus, while today the star Polaris lies approximately at the north celestial pole, this will change over time, and other stars will become the “north star”. Second, the position of the Earth in its orbit around the Sun during the solstices, equinoxes, or other seasonal times slowly changes.

The cause of this was first discussed by Sir Isaac Newton in his Philosophiae Naturalis Principia Mathematica where he described it as a consequence of gravitation. Though his equations were not exact, they have since been revised by scientists and his original theory proven correct.

It is now known that precessions are caused by the gravitational source of the Sun and Moon, in addition to the fact that the Earth is a spheroid and not a perfect sphere, meaning that when tilted, the Sun’s gravitational pull is stronger on the portion that is tilted towards it, thus creating a torque effect on the planet. If the Earth were a perfect sphere, there would be no precession.

Today, the term is still widely used, but generally in astrological circles and not within scientific contexts.

We have written many articles about the equinox for Universe Today. Here’s an article about the astronomical perspective of climate change, and here’s an article about the Vernal Equinox.

If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Gravity. Listen here, Episode 102: Gravity.

Sources:
http://en.wikipedia.org/wiki/Axial_precession_%28astronomy%29
http://en.wikipedia.org/wiki/Chaldea
http://en.wikipedia.org/wiki/Ecliptic
http://en.wikipedia.org/wiki/Great_year
http://www.crystalinks.com/precession.html
http://en.wikipedia.org/wiki/Isaac_Newton

Reference:
NASA: Precession