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SunRise, SunSet, NewMoon, FullMoonCoordinate Systems

2.5 The BestView and InterestingObjects Functions

BestView is used to find when a planet, or any other object, is in a good viewing position relative to the Sun. This occurs when the object is furthest from the Sun in relation to your viewing angle.

Determining the best viewing times for specified objects.

For the inner planets Mercury and Venus, the event dates are the evening and morning apparitions, which indicate when the planet appears in the evening or morning sky. For outer planets such as Mars, Jupiter, and Saturn, the event date is the time of opposition, which indicates when the planet is opposite in the sky to the Sun. For low-orbit Earth satellites, the event date is the transit visible time, which indicates when the satellite is visible above the horizon and is making a transit overhead. For other objects, such as stars and constellations, the event date is simply the transit time at which the object crosses the local meridian line.

Event dates returned by BestView.

A typical use of BestView is to determine when, for instance, Mars is next in opposition.

This shows that Mars reaches opposition on 1993 January 8.

In[21]:=BestView[Mars, {1993,11,17}]


During an opposition, Mars is in the opposite direction to the Sun and consequently the orbits of Earth and Mars are close together. An opposition is a very good time to view Mars as it is at its largest apparent size. Every seventh opposition of Mars is particularly favorable as during those oppositions it is closer than normal to Earth. Mars oppositions are listed in Appendix A.11. In general, when any planet is in opposition, it is visible all night because it rises when the Sun sets, and sets when the Sun rises.

A planet is visible primarily in the morning sky before opposition, and in the evening sky after opposition. Retrograde motion also occurs around the opposition event date. In the case of Mars, retrograde motion lasts about 10 weeks and reverses 15 degrees in the sky. For Jupiter, retrograde motion lasts about 16 weeks and reverses 10 degrees. For Saturn, retrograde motion lasts about 20 weeks and reverses only 7 degrees.

Similarly, you can use BestView to find some good viewing dates for Mercury.

The inner planet Mercury is visible in the evening sky around 1993 October 14 and in the morning sky around 1993 November 23.

In[22]:=BestView[Mercury, {1993,11,17}]


Mercury is a particularly difficult planet to see because it is rarely in a good viewing position. BestView gives you the optimal dates to view it.

When an inner planet is at its greatest elongation east of the Sun as viewed from Earth, it is at its highest point in the evening sky just after dusk; at this time the planet is said to be making an evening apparition. The planet is also furthest from the glare of the Sun, so the time of an evening apparition is the best time for viewing the planet. Before an evening apparition, the planet is visible in the evening sky, whereas after the evening apparition, it quickly moves toward the Sun to reappear later in the morning sky.

Once you have determined an evening apparition date for Mercury, try searching the western sky just after dusk. The best evening apparitions are in spring. You should start searching for Mercury about 40 minutes after sunset, and you can give up by about 70 minutes after sunset. Similarly, once you have determined a morning apparition date for Mercury, try searching the eastern sky just before dawn.

Viewing Asteroids

Only one asteroid is visible with the naked eye, and it can only be seen during opposition when it is at its closest and brightest. BestView allows you to find the date.

This shows that Vesta reaches opposition on 1993 August 28.

In[23]:=BestView[Vesta, {1993,11,17}]


A call to Ephemeris on the opposition date determines the circumstances of the event. You can see that Vesta is 180 degrees from the Sun, and so it is indeed in opposition. Its apparent magnitude is 5.6, which is just visible to the naked eye under reasonable conditions.

In[24]:=Ephemeris[Vesta, Opposition /. %]


Viewing Stars and Satellites

BestView can also be applied to stars, in which case it returns a transit date that is the precise time at which the star crosses the local meridian line.

BestView shows that the star Sirius crosses the local meridian at 04:22.

In[25]:=BestView[Sirius, {1993,11,17}]


The local meridian is the great circle that starts at the point on the horizon directly south of your location, and passes up through the zenith and then down to the point on the horizon directly north of your location. It also continues down to the nadir point directly below you, but that half of the meridian is not visible. The north and south celestial poles are fixed points on your local meridian, although one of the celestial poles is not visible below the horizon.

All stars cross your local meridian twice every day, once at a maximum angle above the horizon, and once at a minimum angle, usually below the horizon. The transit date is the time of the maximum crossing and is, therefore, a good time to view an object.

Another particularly useful application of BestView allows you to determine when a low-orbit satellite is visible. In this case a transit visible event date is returned.

When a low-orbit Earth satellite is crossing the local meridian line and is illuminated by the Sun but the viewer location is still in darkness, the satellite is said to be transit visible. This is a fairly rare event, as most satellites are only a few hundred kilometers above the Earth's surface, and hence normally eclipsed by the Sun when the Earth's surface is in darkness. However, there is a very small window, approximately five minutes wide, when the satellite becomes transit visible.

More details on transit visible event dates are given in Section 7.2.

The InterestingObjects Function

InterestingObjects is a function related to BestView. The BestView function determines a time when an object is in a good viewing circumstance; InterestingObjects takes the inverse approach and returns all interesting objects that are visible at a specified time.

Finding interesting objects above the horizon.

By default, only objects more than 15 degrees above the local horizon are returned. The Altitude option lets you set the lowest altitude above the local horizon to begin the search.

InterestingObjects searches for all planets as far out as Uranus, plus a few asteroids, several dominant constellations, and many bright stars. It also searches through a long list of bright clusters, nebulae, and galaxies. In each case it returns the brightest objects in each category and sorts them according to apparent magnitude.

Here is a list of the dominant objects above the horizon for Melbourne at 22:00 on 1995 December 1.

In[26]:=Print /@ InterestingObjects[{1995,12,1,22,0,0}];

This searches for all the interesting objects more than 30 degrees above the local horizon. It shows that the Moon and Saturn are above the local horizon. It also shows that there are no dominant constellations at this time, although from the earlier output you can conclude that Orion, Canis Major, and Pegasus are low on the horizon. The brightest stars are Canopus, Achernar, and Fomalhaut.

In[27]:=Print /@ InterestingObjects[{1995,12,1,22,0,0},
Altitude -> 30*Degree];

Some interesting open clusters that are built into Scientific Astronomer include ThetaCarinaeCluster, a 2nd magnitude cluster easily visible to the naked eye; PtolemysCluster, a large 3rd magnitude cluster in Scorpius separated by only 4 degrees from the 4th magnitude ButterflyCluster; JewelBoxCluster, a very colorful 4th magnitude cluster inside the Southern Cross; and WildDuckCluster, one of the best open clusters, visible in binoculars as a 6th magnitude fuzzy patch in Scutum.

Two of the many globular clusters built into Scientific Astronomer are OmegaCentauriCluster, easily the finest globular cluster and visible to the naked eye at 4th magnitude; and HerculesCluster, the brightest globular cluster visible from the northern hemisphere, at 6th magnitude.

Some notable diffuse nebulae built into Scientific Astronomer include OrionNebula, a naked-eye emission nebula of 4th magnitude, greenish swirls visible in binoculars; EtaCarinaeNebula, a 6th magnitude emission nebula and the largest diffuse nebula in the sky; and OmegaNebula, a 6th magnitude red emission nebula in Sagittarius.

Notable planetary nebulae built into Scientific Astronomer include HelixNebula, the brightest planetary nebula at 6th magnitude in Aquarius; DumbbellNebula, a large 7th magnitude planetary nebulae in Vulpecula, 0.25 degrees in diameter and green in color; CatseyeNebula, an 8th magnitude green and red planetary in Draco, looks like a cat's eye; and RingNebula, a 9th magnitude planetary in Lyra, separated by 7 degrees from the star Vega.

Many bright galaxies are also built into Scientific Astronomer. The list includes the 3rd magnitude AndromedaGalaxy and the 7th magnitude CentaurusGalaxy. At 2.2 million light years the Andromeda Galaxy is the furthest object visible to the naked eye.

The lists BrightClusters, BrightNebulae, and BrightGalaxies contain many of these predefined deep sky objects that are built into Scientific Astronomer.

BrightClusters gives a list of all the bright clusters, both open and globular, used by InterestingObjects. The list is sorted by apparent magnitude.



BrightNebulae lists all the bright nebulae, both diffuse and planetary.



BrightGalaxies lists all the bright galaxies.



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