Understanding Celestial Motions (continued) Page 4 of 7 Part 2: Planetary Motions The word planet is derived from the Greek word for wandering star. Let’s find out why with an appropriate definition. First, we need to obtain and plot the positions of planets on a star chart. To get the positions, you will need to consult a calendar of astronomical events or an ephemeris. Using a search engine on the Internet, enter the words “Astronomical Ephemeris” in the search window. Find a site that lists the geocentric positions (RA and DEC) of the Sun, Moon, and planets for each day of the year. In the table below, write the celestial coordinates of each of these objects for the date you are doing this lab. Round the RA to the nearest minute and the DEC to the nearest degree. Then plot and label these objects on the star chart on page five: Object Date Symbol RA (hrs mins) DEC (deg) Visibility The Sun The Moon Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto The information below will help you identify certain constellations and bright stars that astronomers recognize: Star Name Polaris Aldebaran Rigel Capella Betelgeuse Sirius Castor Pollux Canopus Regulus Spica Arcturus Alpha Centauri Antares Deneb Do you see anything noteworthy in your plot of the planets? Suggest an explanation for it. Constellations of the Zodiac: Aries Taurus Gemini Cancer Leo Virgo Libra Scorpius Sagittarius Capricornus Aquarius Pisces DEC deg +89 +19 +39
The Correct Answer and Explanation is:
In this lab, you’re working with the concept of planetary motions and their positions relative to the stars and constellations. The first step is to understand how celestial coordinates, specifically Right Ascension (RA) and Declination (DEC), define the positions of celestial objects in the sky.
To start, you will need the RA (hours, minutes) and DEC (degrees) for each of the planets, the Sun, and the Moon from a reputable Astronomical Ephemeris. This is a table that shows the position of celestial objects for each day of the year, based on the geocentric view (the Earth-centered view). Websites like NASA, the US Naval Observatory, or astronomy databases can provide this data.
Steps:
- Consult an ephemeris: Search online for “Astronomical Ephemeris” and find a table listing the positions of the Sun, Moon, and planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto).
- Record RA and DEC: From the ephemeris, write down the RA and DEC for each object, rounding to the nearest minute for RA and the nearest degree for DEC.
- Plot on a star chart: Using the RA and DEC coordinates, plot the positions of these celestial objects on a star chart. You will also be able to mark the constellations (like Aries, Taurus, Gemini, etc.) and major stars (such as Polaris, Aldebaran, etc.).
Key Observations:
- Planetary motion: Planets tend to move along the ecliptic plane, which is the apparent path of the Sun through the sky. This means that, when plotted on your chart, most planets will appear near or along the zodiac constellations.
- Wandering stars: The term “wandering stars” comes from the fact that planets don’t follow the same predictable path as stars. They move in a way that seems irregular relative to the background stars. This “wandering” is due to their elliptical orbits around the Sun.
- Noteworthy patterns: Depending on the date of your observation, the positions of the planets will form different patterns. You might see clusters of planets near certain constellations or particular alignments that could be linked to astrological significance.
Suggestion for Explanation:
- Zodiac alignment: Planets and the Sun typically pass through the zodiac constellations, which is why we associate them with astrology. The path of the planets along the ecliptic can form patterns within these constellations, and at certain times, multiple planets may be visible in the same part of the sky, creating a planetary alignment.
- Retrograde motion: Planets like Mars, Mercury, and Venus sometimes appear to move backward in the sky. This happens because of the relative positions and motions of Earth and the other planets. When Earth “overtakes” another planet in its orbit, the planet appears to reverse direction momentarily.
In your plot, pay attention to how the planets move in relation to the constellations and whether they exhibit retrograde motion or alignments.
