Activity 6 – Planet Hit List

Context

There are five planets that are bright enough to be seen without telescope or binoculars. They are Mercury, Venus, Mars, Jupiter and Saturn. Unfortunately planets cannot be printed on a Star Wheel because their positions on the wheel change from day to day. In this activity you will learn how to locate these planets on your Star Wheel.

Specific Learning Outcomes

You will use your Star Wheel to locate and identify all five planets: Mercury, Venus, Mars, Jupiter and Saturn.

Teacher Planning and Preparation

Seeing all five planets is only rarely possible in one night. It might take up to six months before you can get a sighting of all five of these planets. Seeing all five planets is obviously not a classroom activity, although it makes an excellent long-term homework project. Give your students the Planet Hit List record sheet so that they can keep a record of their sightings.

Although you cannot see the planets in the classroom, you can teach how to find the location of a planet and how to find out when a planet will be visible using the instructions below. Get students to pencil in today’s locations of all five planets on their Star Wheels, and to work out which planets are visible tonight. Then get them to work out when each planet will next be visible in the night sky and to make a plan for seeing all five planets.

What You Need

• Star Wheels (see Activity 3 – Make a Star Wheel)
• The appropriate pages from the Ten Year Ephemeris (Download here)
• Planet Hit List record sheet (Download here)

Science Background Knowledge

Why don’t planets have fixed locations on the star wheel?

Earth is constantly on the move as it races around the sun once every year. (To clarify Earth’s movement see “Rotate or Orbit?” in Activity 2.) The other planets also orbit the sun, but at different speeds. As a result the planets appear in different directions at different times when viewed from Earth. Because both Earth and the other planets are moving, the result is quite a complex pattern of movement that is not the same from one year to the next.

The stars are also constantly on the move as they race around the centre of the galaxy. However, they are so far away from us (the closest star is about 5,000 times further away than the furthest planet) and the journey around the centre of the galaxy takes so long that it takes hundreds or even thousands of years before we can see significant changes in the locations of the stars. Yes, you will need to get a new Star Wheel every thousand years or so, but for the present time we can treat the stars as fixed.

A simulation of the planets orbiting the sun is called an Orrery. It enables you to see the planets’ orbits. An excellent online Orrery is the Schools’ Observatory Orrery. Another is Karen Strom’s Orrery.

Visit one of these online Orrerys. Imagine that there are stars all around the edge of your computer screen. The stars are not moving. As the planets move you will notice that they can be seen in different directions at different times. Each star stays in the same direction, so the planets appear to wander among the stars. (The word planet comes from a Greek word meaning wanderer.)

Why are planets always found close to the ecliptic?

When the Solar System formed it evolved from an almost flat spinning disc. As a result, the planets all go round the same way, more-or-less in the same plane. This plane is called the plane of the ecliptic. When we look out from planet Earth, we see this plane as a line across the sky called the ecliptic. The planets always appear somewhere along this line.

The planets are all more-or-less in the same plane. They vary from that plane by only a few degrees (with the exception of Pluto which has an orbit 17° from the ecliptic). You will always find the five planets Mercury, Venus, Mars, Jupiter and Saturn within a few degrees of the ecliptic. These planets are all so bright that even on the occasions when they are up to 5° off the ecliptic you will have no trouble identifying them.

See a picture of some planets lined up on the ecliptic here.

How bright are the planets?

To give you an idea of how bright the planets are, here is a list showing the 20 brightest celestial objects, arranged from brightest to dimmest.

1. Sun
2. Moon
3. Venus
4. Jupiter
5. Mars
6. Mercury
7. Sirius (brightest star in the night sky)
8. Canopus
9. alpha Centauri (the pointer furthest from the Southern Cross)
10. Arcturus
11. Vega
12. Capella
13. Rigel
14. Procyon
15. Achernar
16. Betelgeuse
17. beta Centauri (the pointer nearest to the Southern Cross)
18. Saturn
19. Altair
20. Aldebaran

Planets vary in brightness over time according to how far they are away from the sun, and according to how much of the bright side of the planet is facing towards Earth. This list has been made using typical values for brightness. Jupiter is usually brighter than Mars, but at times Mars outshines Jupiter.

Mercury and Venus both orbit the sun closer than Earth. This means that both of them are never seen far from the sun – they appear only in the evening after sunset, or in the morning before sunrise.

Betelgeuse varies in brightness over a period of several years. At its brightest it is brighter than Procyon, at its dimmest it is dimmer than Aldebaran. It is placed here according to its average brightness.

Sirius is the brightest star in the night sky. If you see something that looks like a star that is brighter than Sirius, then it must be a planet.

Do planets twinkle?

Stars twinkle because their light is bent as it comes through the air. Turbulence causes them to dance about a little and to get brighter and fainter. This is called twinkling.

Planets do not twinkle as much as stars because, being closer to Earth, they appear as very small disks instead of points. The effect of turbulence is not as noticeable for a disk as it is for a point light source.

Planets do twinkle, but they twinkle much less than stars do.

By the way: Close to the horizon, especially in winter, you will see bright stars flash vivid red and green. This beautiful effect is an extreme form of twinkling, but it is something that planets do not normally do.

Classroom Lead-In

You can prepare students for planet hunting in two ways.

Research

One way is to research what the planets are like, and what they look like in the night sky. Some useful Internet references.

Role play

The other way is to explore how the planets move, and why they appear in different places on the Star Wheel at different times. An excellent way to do this is as a role play.

Make sure you have a clear space in which students can move. Have one student in the centre of the space to be the sun. Have a further nine students to be each of the nine planets. Place a large circle of students surrounding the sun and the planets to represent the constellations. It works best when you can get as much space between the planets and the stars as possible so the outer circle needs to be as big as possible. If space is limited you might want to reduce the number of planets that you represent.

Get each of the students in the outer circle to choose a constellation. For example one student might choose to be Orion, another might choose to be Scorpio. It is not necessary to arrange them correctly because this simulation demonstrates the principles of planetary motion, not the specifics. However, it is best to restrict their choices to constellations that are actually close to the ecliptic ( for example, the Zodiac constellations) because planets can never be found in constellations that are away from the ecliptic. The students in the outer circle do not move.

Instruct the ‘planets’ to orbit by walking around the sun. So long as you have arranged the planets in the right order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, etc, and so long as each ‘planet’ walks at about the same speed and keeps the same distance from the sun, they will end up orbiting in roughly the right way. Now get ‘Earth’ to call out where each planet can be seen. For example: “I can see Mercury in Orion and Mars in Scorpio. Now Mercury has moved to Taurus…”

Instructions

How to find the location of a planet

1. Look down the column of dates in the ephemeris until you find the closest date to today’s. Look across to the column for the planet you are interested in. The number that you find tells you how far around the ecliptic the planet is today. (The number is called the Right Ascension of the planet in degrees, but you do not need to know that to find the planet.) As an example, we show Venus on the 21^st February 2003. The number is 290.

2. Take the wheel out of the pocket of your Star Wheel and find the same number on the outermost ring of the wheel.

3. Using a ruler or any other straight edge connect that number with the celestial pole (either the Pole Star or the South Celestial Pole) at the centre of the wheel.

4. The planet will be where the straight line crosses the ecliptic. You can mark it in with a pencil if you like.

5. Now that you know the planet’s location on the wheel, put the wheel in the pocket and set the Star Wheel to see if the planet is in the night sky and where to find it. (Instructions on setting a Star Wheel)

How to find out when a planet will be visible

To find out when a planet will be visible you need to look ahead in the ephemeris and plot the planet’s location on your wheel for different dates. Keep going into the future, setting your Star Wheel for each future date, until the planet appears in the night sky.

Follow Up and Extension

When students complete the Planet Hit List record sheet, this achievement can be acknowledged with a certificate.

Downloadable Resources

	Ten Year Ephemeris
	Planet Hit List record sheet - worksheet
	Certificate
	Help with printing and downloading