A profound and poetic doctrine that was clarified with the restoration of the gospel of Jesus Christ involved a few verses written by Paul the Apostle:
"There are also celestial bodies, and bodies terrestrial: but the glory of the celestial is one, and the glory of the terristrial is another. There is one glory of the sun, and another glory of the moon, and another glory of the stars: for one star differeth from another star in glory." 1 Cor. 15:40-41
In Corinthians chapter 15, Paul was trying to explain the doctrine of immortality. This doctrine is hard for our mortal brains to comprehend, much like the concept of having a physical body was probably difficult for us to understand in the premortal life. As Paul explains, our natural, "earthy" bodies will be changed when resurrected into something "heavenly"; the spirit and the body will no longer be distinguishable, but will become one - a "spiritual body". There has been much debate among Christians about the nature of God in relation to whether or not He has a body. Paul clarifies that the Lord does indeed have a body, for lack of a better word, but goes on to explain that it's something transcending our current understanding. When resurrected, Jesus Christ made a point to show His disciples that He had indeed overcome death, emphasizing that He wasn't merely a spirit by letting them touch His hands and feet. He then reinforced His physicality by eating fish and honeycomb (Luke 24:36-48). He was showing the disciples that He had completely overcome death, both physically and spiritually, as He stood before them as a perfected Being. All that is good about having a mortal body will be restored and perfected when we are resurrected.
I'd now like to explore what Paul touches upon in verses 40-41. Here Paul introduces us to the logical truth that there are different glories. There are varying degrees of faith, valiance, and understanding for each son and daughter of God. Likewise, there will be different degrees of glory assigned when we meet with the Lord on Judgment Day. This assignment will be merciful and just, reflecting our true desires; having a perfect understanding of ourselves we will be our own judges.
Paul mentions the celestial and terrestrial glories. Through the Prophet Joseph Smith, the Lord expanded our understanding about this doctrine and introduced a third glory to match Paul's "sun, moon, and stars" metaphor: the telestial glory (D&C 76:50-113, D&C 88:20-32). I've pondered this doctrine quite a bit and have gained somewhat of an understanding about what these symbols and metaphors may mean.
Celestial - (Latin: caelestis) "heavenly, pertaining to the sky"
The celestial glory/kingdom is represented by the sun. The sun is the brightest object most people can relate to, and is an appropriate metaphor to use when trying to explain the glory of God the Father. The sun is our source of light and life. Without it there would be no possibility to sustain life on earth. Jesus Christ, the Son, is the source of light and life for our eternal souls. Our solar system revolves around the sun, which I like to think parallels how our lives should be centered around God and His truth. When we keep the first and great commandment (Matt 22:36-38) everything else will work together for our good. All else will fall into the right order when God is our first priority. One way to appropriately show our devotion to God is by setting apart the first day of the week as a day of concentrated worship. Though I don't think Sunday's name was dedicated specifically to express this (the word Sunday derives either from Egyptian astronomy or Roman paganism), I like it anyway. The Latin word caelestis is also used in reference to the sky (consider the Spanish word cielo, also meaning "heaven" or "sky"), placing celestial things symbolically above the earth. Likewise, to obtain a celestial glory we must consciously let our souls be converted to a state that ascends above our natural state of worldliness.
Terrestrial - (Latin: terrestris) "earthly"
The terrestrial glory/kingdom is represented by the moon. It might at first seem incongruous that the "earthly" glory has the moon as its symbol, but the meaning works out brilliantly when considered carefully. First of all, Paul mentioned the moon to introduce a body of light that's less glorious than the sun. The moon produces none of its own light; it is a reflector, gathering all of its light from the sun. The moon orbits around the earth, waning and waxing in its shadow. "Behold, many are called, but few are chosen. And why are they not chosen? Because their hearts are set so much upon the things of this world..." (D&C 121:34-35). In contrast, the earth orbits around the sun. Per D&C 88:25-26, "the earth abideth the law of a celestial kingdom, for it filleth the measure of its creation, and transgresseth not the law - wherefore, it shall be sanctified". Another interesting scripture to consider is Helaman 12:15: "...it appeareth unto man that the sun standeth still; yea, and behold, this is so; for surely it is the earth that moveth and not the sun". Worldliness and the natural man cause us to move away from God. Sometimes prophets tend to say that when we sin the Spirit withdraws itself. I think it makes more sense the way King Benjamin described it in Mosiah 2:36 when he said that when we sin we withdraw ourselves from the Spirit. We have the gift of moral agency; we are endowed to decide where to go and what to become. We can move away from God or towards Him. Heavenly Father won't force us to participate if we do not desire eternal life. But He'll always be there for us, constant as the sun, no matter what we decide.
Telestial - (Greek root: tele) "far, far off, at or to a distance"
The telestial glory/kingdom is represented by the stars. The word "telestial" is unique to Mormonism. If you Google the word you'll be taken to a bunch of resources referring to the LDS Church. Joseph Smith was inspired to associate this word with "the glory of the stars". Paul chose stars to symbolize the least degree of glory because they appear (from our distant perspective) to have even less light than the moon. The word telestial denotes an element of distance. Think telescope, telephone, telegram, teleport - all of these things involve being able to do something from a distance. Those whose hearts are distant from God will receive a telestial glory, "for how knoweth a man the master whom he has not served, and who is a stranger unto him, and is far from the thoughts and intents of his heart?" (Mosiah 5:13).
I think the Lord's prophets often receive revelation by studying patterns in the physical world. Like them, we can receive new revelation when we remember that "all things denote there is a God" (Alma 30:44).