הן קרבו ימיך למות, "here your days to die are drawing near.⁠" According to the plain meaning of the text this verse teaches that the period man has to live has been predetermined, every single human being having a time allocated to him at birth. God sees to it that every person lives out the time thus allocated to him at birth. This would correspond to Exodus 23,26: "I will ensure that you complete the number of your days (years).⁠" The expression קריבה, "approach,⁠" mentioned here is one which is specifically reserved for the approach of death of righteous people as we know already from Genesis 47,29 when the Torah speaks about the approaching death of Israel our patriarch. Similarly, Kings I 2,1: "the time for David to die approached.⁠" These verses prove that such times had been determined by God already at birth, and that God's השגחה, "benevolent supervision of history,⁠" ensures that the individuals in question get to live out their allotted time, that their lives are not cut short by acts of violence or natural disasters. Alternatively, God lengthens the allotted life span of these people as a reward for merits which they have accumulated during their lives. At the same time, their allotted life span may be reduced due to demerits they have become guilty of. The important thing is that no one dies unless God has so determined.
According to our information that God has empowered planetary forces, stars and their constellations, to exercise their influence on the lives of people on terrestrial earth, and the orbits of these forces are of constructive or destructive influence on these lives so that their lives appear to depend on mazal,⁠" i.e. such influences (Targum Kohelet 9,2), we find that our sages did not deny the impact of such mazal; we therefore seem to be faced with a dilemma here. Our sages said specifically that phenomena such as economic success, (duration of) life on earth itself, as well as the ability to have children, do not depend on one's merit but on one's mazal (Moed Katan 28). If that is so, where is there room for God's השגחה in the matter? Further details of our sages' view is found in their statement that though both Rabbah and Rav Chisdah were righteous individuals (seeing that when they prayed for rain it materialised), one lived only to the age of 40 whereas the other lived to the age of 92. Rabbah's 40 years on earth were filled with painful experiences. Rav Chisdah was blessed to marry off 60 children, whereas Rabbah buried 60 family members. The latter could not always afford to eat even bread made from barley, whereas the former fed his dogs with white bread. These statements are proof positive that the sages acknowledged the influence of mazal in our lives, i.e. that there is substance to the discipline we call astrology.
It is true that these planetary phenomena have been equipped with initial power to influence our fates. However, they are not supreme. Their influence need not be final. They are subservient to the Lord Almighty, to the attribute Hashem. If people born under adverse mazal turn to the supreme God, to Hashem, and appeal to Him to neutralize their apparently negative mazal, God will respond to such prayer if it is warranted. In fact, we may go further and posit that God instituted mazal partly in order to encourage us to pray to Him to change adverse mazal in His benevolent way. We know that God can lengthen our lives from the very verse in Exodus 23,26: "I will make full the number of your days,⁠" i.e. "I will add to them if the situation warrants.⁠"
King Chizkiyah had 15 years added to his life expectancy although the prophet Isaiah had been sent to him to prepare his last will and testament as he was not going to recover from his sickness. God responded to his prayer and revoked the decree he had been subject to (Isaiah 38,5). We also know that when the occasion warrants it God reduces the number of years allocated to a person at birth, from Achazyah King of Israel son of Achav. The reason he had his life span shortened was that when he fell ill, instead of turning to God or to an accredited prophet, he turned to idols to inquire about his chances of a recovery (compare Kings II 9,16). He was told clearly that he would die now because he had turned to Baal Zevuv, the god of Akron, ignoring the presence of accredited prophets in Israel. The verse we just quoted makes it plain that Achazya's death at that time was due to his having turned to idols at that time. Had death already have been decreed upon him previously, what point was there in the prophet telling him about it now? The above examples teach us that God reserves the right to lengthen or shorten the life span decreed for man at birth. God remains a free agent; this is why He is known as שדי. The word, derived from שדד, means that He is in charge of the laws of nature, changing them at will if He so desires. (Compare author's comments on Genesis 17,1) These adjustments in life span, etc., occur with a view to reward and punishment. It follows that the statement that "life (expectancy), children (ability to have them), and economic success, do not depend on merit but on mazal,⁠" which we quoted from Moed Katan, means that the power of prayer is so great that it can override predetermined fates set according to mazal. The astrologers themselves admit that the general rules are set by God, whereas the planetary systems only supplement details pertaining to the lives of the subjects under their influence. Clearly, He who sets the general rules is able to set aside mere details, i.e. the subordinate powers assigned to the horoscopes. It follows that it is within man's power to create the conditions for setting aside such details if his actions merit it and he appeals to the Lord to use His powers on his behalf. On occasion, God fulfills man's desire without waiting for man to appeal to Him. What the statement in Moed Katan, regarding the three cardinal parts of man's fate does mean is that in these matters God does not interfere with the predetermined fates of these individuals unless asked, unless appealed to. This is the meaning of Psalms 145,19 ואת שועתם ישמע ויושיעים, "and their cry He hears and He delivers them.⁠" On the other hand, the same psalmist says in the first half of this verse: "He does the will of those who fear Him;⁠" this refers to the unspoken requests by these people. We have proof positive from the Torah and the Books of Prophets that the three critical values of man, 1) children, 2) life, and 3) economic success, פרנסה, are all subject to prayer. Rachel, who had been unable to conceive and had prayed was granted children (Genesis 30,22): "God listened to her and opened her womb.⁠" King Chizkiyah provided proof for the fact that God changes even decrees of death announced by one of His prophets at His command. Elijah and Elisha both experienced that the supply of food was dramatically increased through their prayerful intervention in the fates of starving individuals (Kings I 17,13-15 and Kings II 5,42-44). These individuals were saved from famine, although clearly that had been their assigned fate.
This is the reason why our sages singled out these three aspects of life which are normally dependent on mazal to inform us that even these matters are subject to change by means of prayer by the intended victims.

A Midrashic approach (based on Devarim Rabbah 9,4). The words: "here the days of your death are approaching,⁠" reflect a conversation between Moses and God. Moses had said to God: "I have praised You using the word הן (Deut. 10,14), and You are using the same word introducing the decree that I am to die.⁠"
We may understand this by means of a parable. A king had an outstanding minister, one of whom he was exceedingly fond. One day this minister came across a sword, which was exceptionally beautiful, the like of which he had never seen. He immediately determined that this sword was appropriate for his employer, the king. What did he do? He bought it and presented it to the king as a gift. The king said: "may they cut off his head with it.⁠" This is what Moses had in mind when he said to God: "are You going to use my gift to turn it against me?⁠"
Another thought which is contained in the words הן קרבו ימיך למות, is that God meant: "the time has come for your sun to set and for the moon to shine.⁠" This was a reference to the statement of our sages we have already quoted in which the face of Moses was compared to that of the sun whereas that of Joshua, his successor, was compared to that of the moon. When God said to Moses: "here you are going to lie with your ancestors, and this nation will arise and serve idols, etc.,⁠" (verse 16 in our chapter), He meant that "your power, that of the exceptional level of prophecy granted to you, will continue to illuminate the moon (inspire Joshua) although you are already in your grave, just as the sun even after it has set shines upon the surface of the moon to enable it to reflect its light.⁠" The word וקם may be read together with the words הנך שוכב עם אבותיך וקם, "although you are lying with your fathers it will rise.⁠" Just as the sun supplies light to the moon even after it has set, you Moses will inspire Joshua even after you have joined your fathers. This is also the meaning of the word ואצונו, i.e. "I will impress My power on the moon.⁠"

וילך משה ויהושע, "Moses and Joshua went; from the camp of Israel to the camp of the Shechinah.